What time of year are you planning to go? Campers are much more expensive to hire in Iceland in high season than most other places, and many people look at the price and change their mind. Though indoor accom is also difficult in high season.
I've been told 1 week should be enough to circle the island
10 days is a more practical minimum, and 2 weeks would be better. It's 1350km round the ring road, plus 50km each way to the airport, plus extra for detours to see stuff. So it tends to add up to the best part of 2000km so if you want to drive 250-300km a day that's up to you. In a week, I'd suggest concentrating on a smaller part of the island, especially with children, there's plenty to see.
watching puffins
Come May-July. Not available other times, not just in Iceland.
wouldn't go out of my way for it
You can see them at Vik which I expect you'll probably visit without going out of your way. But if you do decide to spend your week on visiting the Westfjords, and rather than driving all aroudn the island, and come at the proper season, it really is worth going out of your way to see them at Latrabjarg.
watching working volcano (lava stream),
Not available in Iceland. I think Hawaii is just about the only place on the planet you can reliably see this.
working geyser
There is precisely one that can be relied upon. It is at a place called Geysir and you'll probably go there, and it erupts every 5-10 mins.
waterfall at pinvellir park
Actually the waterfall at Thingvellir is only a little one. You probably have in mind Gullfoss which is usually visited on the same day as Thingvellir when people go on the organised Golden Circle tour, though in your own vehicle that isn't compulsory. It's only 8km from Geysir.
whale watching trip along the way
If you don't get to Husavik, and you don't really have time to get there in my view, then there are trips in Reykjavik. But unless your kids like going on boats for the sake of going on boats, they might find this a bit boring, because you only see the sea mammals rather briefly, and often just a dorsal fin briefly showing. Watching puffins walk 3 feet from your nose on a cliff top at Latrabjarg is actually much more engaging for most children.
Also: are roadmaps widely available
Yes, there's even a choice of different qualities.
have never travelled by camper but I am assuming you will regularly need to go to a camping or special camper place to fill up on water, get rid of dirty water and the like.
Nor have I, but I expect you can fill up on water at a normal filling station, though you will need to go to special facilities to get rid of the dirty water. Your hirer will doubtless be able to advise you on all this. There are a large number of campsites in Iceland, most villages have one, plus other key tourist locations, though the simpler ones do not have facilities for draining camper vans. Out of season things may be trickier.
