September is a good time to travel. Most European schools go back at the start of the month, freeing up all the hotels, resorts, theme parks, etc.
This is how I suggest it; Fly into London. See that. Take the Eurostar to Paris. Visit Paris and only THEN rent the car.
First, you'll save on the rental time. You really don't want a car, let alone need one, in London or Paris. I lived in both cities car-free so trust me on that. I've also taken much younger kids to both cities alone without a car and it was the best plan.
I would recommend taking a trip through any other part of Europe that's reasonable to see (i.e. Czech Republic, Switzerland, Austria, etc.) and ending up in Rome. Give up the car when you arrive and plan to fly back up to London. Yes, it's probably smarter for your budget. I think Ryanair does Rome-London from alternate airports. Ciampino Airport is right near the center of Rome so more convenient than DiVinci. You would fly into Standstead which is quite far from London but there are frequent, direct trains so it's not a problem. You can't really go between Standstead and Heathrow and never mix reservations on the same day anyway. Plan to stay at least one night on the way back.
Last summer we drove from where we live in Eastern France, down to the Amalfi Coast. We stayed in Ravello, which was amazing. I highly recommend it.
I also agree that kids love Venice with the canals. Florence was a different story. Boring museums, lots of walking, not kid-friendly at all. You might want to organize your visit (I'm not saying don't go!) with this in mind. One parent do something with the kids while the other take the grandparents to a few museums?? Rome was a winner.
If you go to the Naples region, which I recommend, you probably don't want to stay in Naples itself. Don't stay on the Amalfi Coast and expect to see Pompeii. My mistake! If you want to go to Pompeii, stay in Saleno or somewhere else close by. Yes, on the map, it looks close but it turns out it was nothing but terribly winding roads down there. Two of mine are very carsick-prone. My Greek-mythology craving son was very disappointed but it just wasn't do-able. Stay two nights right there. The ruins are very tiring and it will take up a day. Then head up to Amalfi and relax.
I also want to talk about Disneyland. I'm from California, have been to the parks in Florida too, and we took our kids once to Disneyland Paris. We've avoided it since. Thank goodness they didn't ask! Honestly, if you can, or have been to the other parks, skip this one. It'll be a waste of time.
There are a few theme parks, that will still be open in September, that you may want to read up on. We take our kids to Europark, which is much less busy and more fun for them. Less waiting in line. It's European country-themed so you "go to" Spain, Greece, etc. Educational! It's in Rust, Germany, near the French border. Look into other theme parks, perhaps ones that you can't find where you live.
Not to make you paranoid but make sure everyone in your party is aware of pickpocketing and their tricks. Stopping to see street performers is great but be aware. You will be targets because of how many of you there are. The passports must be kept in a hotel safe and don't take them out unless you are traveling. That's the real prize! Precautions like only one of you take a credit card when going out, not carrying large amounts of cash at a time and being sure valuables are tucked away somewhere safe.
My husband's credit card was used to the tune of 10,000 Euros while we were in Ravello. Obviously, he wasn't stuck with the bill (they didn't have the three digit code on the back so he was covered) but the card was in a safe the whole time. We still don't know how the number was extracted. Perhaps talk with your bank. My husband now has a security code that is sent to him via SMS every time a big purchase is made, to be sure he's the one using the card. Maybe set something like that up before, so you don't get stuck like he was! You'll probably be talking to your bank anyway, about which machines are best to take out money and what to do with lost/stolen credit cards.
When you calculate costs, be sure not only to factor in gas but road tolls. They're really steep in some places. One of the reasons why I recommend the car after leaving Paris!
Have fun planning!
