Family Trip In June 202

Thank you so much for that info. I will definitely look further into the rail pass. I just figured that was the easiest way around and what most people did. I agree about Germany and hopefully she will be able to meet up with us.

For what it's worth, here are some kid-friendly ideas for London.
My kids LOVED the London Eye - which came as a bit of a surprise, as I thought it overpriced and underwhelming in my previous experience. But it's got lots of interactive stuff (for instance, the ability to zoom in on a specific building and identify its key characteristics) and it is a unique perspective. Try and book for around sunset, which will give you the best Instagram-worthy photos ... very important for certain members of the family ...
Camden Market was a HUGE hit. They loved the diversity of street food and the vintage stores and the general quirkiness. Combine this with a barge trip down from Camden Lock to Little Venice (an easy walk from Paddington Station), which is an underrated perspective on a part of London that many tourists overlook.
They also really liked the Jack The Ripper walk through the East End of London - now rather desirable, but then a slum, and an ideal stalking ground for a serial killer. Very atmospheric and informative (just don't let your little one look at the photos, as they're rather disturbing).
The Tower of London is excellent, and you could combine this with a trip on the Docklands Light Railway from Tower Hill down to Royal Victoria, and then take the aerial cableway back over the river to Greenwich (your Oystercard is valid for this). A boat trip down to Greenwich is also nice, espeically if the weather is good.
A day walking around Westminster (Houses of Parliament, Westminster Abbey, Westminster Cathedral) and the neighbouring areas (Leicester Square, Trafalgar Square) is great. Museum options in the broader area (most of which are free in London) include the National Gallery, National Portrait Gallery, the Imperial War Museum and the Florence Nightingale Museum, and the Brisih Museum sligthly further north in Bloomsbury. Dedicated shoppers will want to vsiit Oxford and/or Bond Street for reasons that are way beyond my ken ;)
Covent Garden precinct is a crowd pleaser and a walk through Borough Market will appeal to foodies.
Also a West End show is a must. There are agencies that deal in discounted/last minute tickets which I use whenever I travel to London (which is every few months) and I seldom pay more than 25GBP.

No, not a car if you are going city to city. Cars are fine for rural destinations but more nuisance than they're worth in cities. Parking is difficult to find and expensive, one way systems and traffic exclusion zones common, and congestion as good as anything the US can offer.
And agree about train passes - worth doing the maths. You need to look at the individual train operators, and booking 3 months in advance for long distance train tickets will give you discounted prices, a bit like the way airlines work. If you have your itinerary decided by the end of March, book the trains when you book your hotels.

six of you in a car will not be great. In fact you will need a minivan. The tow youngest will need car seats and every diver will need an IDP. Stick with a train ride or two. and cut down on journey times to allow you to see something of where you are visiting.

Six people will need a minivan, six people with luggage will need a large van. Plus one-way international rentals don't work.

@rewolfnus3833293 — you say that that the family has a wishlist, but what's on it? Which are the mandatory stops? Why are there three destinations in Spain? Why London, other than the comfort of an English-speaking city?

I agree that staying in fewer places and possibly doing a day trip from each would be more successful for a family. Maybe London, Paris, Bilbao and Barcelona over your 16days? Or perhaps include a smaller place instead of one of those. In your shoes, I'd get you and your partner to choose the places you'll stay in and then ask the kids what they'd like to see in those places. Otherwise, it's too big an ask of them and you could easily end up with too many places and some disappointed faces.

Train travel would be heaps better between major cities and towns than driving. Faster, more convenient, more relaxing and more fun if you're family don't often use trains. The seat61 site is great and advance purchase tickets can be heavily discounted. One thing we found was that it was sometimes cheaper to buy the kids discounted adult tickets than child tickets on the French railways, so look out for that.

If you're not already aware, major cities like Paris and Barcelona are becoming much stricter about unlicensed apartment rentals to tourists. Don't assume that because it's on a major website that it's licensed. If you want to stay in apartments, take the time to make sure your apartment meets that city's regulations.

We took our kids to Europe for the first time when they were 9 and 12, typically staying about 4 nights in each place - we had a wonderful time and you will too, I have no doubt!

Meant to add, a good alternative to travelling all the way from London, through Paris to Spain, may be to cover a much smaller region at a more leisurely pace. This allows everyone to learn a little more of the language, food, customs and culture. And is usually more relaxing and just as rewarding.

I think that simply getting all 6 people up, washed and out the door is going to be a daily logistical challenge. Enough so that it's likely to cut into your sight-seeing time.

The fewest destinations you can pare this down to--and the closer they are to each other--the more time and money you'll save. I would seriously consider making this an "all-Spain" holiday or a UK and France holiday, depending on your priorities.

I strongly suggest you research travel times (and costs) between destinations before finalizing any decisions. Also bear in mind that those travel times will be station to station (or worse, airport to airport) and not door to door.

I predict that at least one person in your party is going to spend Day 1 in a jet-lagged daze. Maybe more than one. Maybe all of you. What does that suggest about the viability of spending "a day or two" in London at the very beginning? The questions raised by such short stops in such truly major cities are: to what end? To accomplish what?

If your goals are to do anything more than to gather a first, hopefully accurate impression (or see the 2 jam-packed sights that everybody else with only two days is visiting), they are unlikely to be met. Also remember that being constantly surrounded by other tourists isn't going to tell you much about the flavor of the destinations themselves.

Train is the way to go...first get the plan together, you have all winter to tackle the logistics.

AS for the 4 big cities, 4 days/3 nights / minimum is each. 4 nights each would be ideal.

Have German friend meet you in France.

Maybe a stop in wine country or south of France, on way south 2 nights...relax a bit before Spain.

You could spend all this time in London and France easily as well. Spain trip within a trip, not easy getting 6 people going non stop city to city...just saying, you are not a solo backpacker being care free...

Your profile says you're from Milwaukee.
Mainz is almost exactly as far from Paris as Milwaukee is from Minneapolis.
About 330 miles/530 km. Just to give you a sense of scale.

Not exactly a quick jaunt.
Especially if your next stop is Spain.

So, if the shoe were on the other foot and a family of 6 you knew from Europe was visiting Minneapolis (Twins fans?) but all their other stops were much further west, would you be more angry or understanding if they didn't detour to Milwaukee?

Or: would you make the 10-hour round trip to visit them?
Depends on the friendship, I suppose...

If you wanted to, you could prioritize visiting your friends. And it might be very nice indeed to get an "insider's" perspective of their respective cities and regions. But that sounds more like a second or even third trip to Europe.
Because it means de-prioritizing--well, probably eliminating--a couple Big City stops elsewhere.

Sorry, but with 16 nights, you are going to have to make some tough choices.
Whatever you do, don't assume that because something is physically possible (which your itinerary is) means that it's likely to be worthwhile and rewarding.