My kids are age 8 and 10, but started backpacking with my wife and I at age 4 and 6. We used two 2 man floored, single door, double wall tents and it worked out very well – except for the 3-4 pound weight penalty and really hard time fitting two tents into my pack along with the rest of my gear (and 1 extra sleeping bag + much of the kids clothing into my wife's pack). Now that I'm getting older – and the kids cloths are starting to take up more room, and food is also starting to take up more room since they eat more – I'm going to have to find a solution that takes up less pack space (and lighter would be nice too).
Thinking back on our trips, I can't remember any areas that we couldn't have found enough space to pitch a 4 man tent. Typically, we were hiking established trails (Shanendoah valley in VA, Maryland park trails, and the AT in Southern PA), coastal areas, grand canyon, etc. We do a fair amount of bushwacking (the kids also do Orienteering meets) and go climb any hills comprised of sets of boulders we see – but typically camp somewhere "allowed". Even when bushwacking though, we could have found a spot to pitch a 4 man tent within a 10 minute walk of any point. (We have deciduous forests here with medium amounts of brush & undergrowth).
Two tents is extra fun for the kids once they are about 8 years old since they can then pitch it themselves. However, floorless is BIG problem for kids (and my wife) the first time there are spiders – and for the next 10 times you use the shelter after that…
Two tent's are also nice for the parents (we each take 1 kid) since there are only half as many squirming, kicking bodies to wake you up at night. Kid's also tend to need bio breaks in the middle of the night – but 2 kids don't have the courtesy to coordinate their needs. ie: The kids take turns needing to waking you up so they can go water the bushes. By splitting up the kids, each parent only gets woken up half as much :)
I'm personally thinking of going to two tarp tents this year. It looks like the pack space and weight is about the same as 1 of my SD Lightnings. The single wall / condensation aspect makes me nervous – since I'm also thinking of switching the family to down bags this year…

