Although if it was me and it was "just a phase" and you'd rather do two two person tents - I might camp out in the back yard and set those things up with a silnylon tarp between them facing each other so the kids can see and talk to each other in seperate tents. if that works and everyone has fun, do that in the field.
A Big Agnes Fly Creek UL4 is only 4 lbs. for a decent shelter that's very dry and bug proof. I think I'd set up a tarp out in front of it as an extended vestibule, but it'd serve your purpose. My UL2 has survived lots of rain ,wind , sleet and snow and performed very well. If you get the footprint, you can do a fast pitch with it and the fly and have a 4 person shelter that's 2 1/2 lbs.
speaking from experience, (I use a very similar ex-NOLS pyramid tent for this role) the reason I'd reccomend the shangri-la or a similar 4-5 person pyramid tent is that at 5 pounds they aren't a lot heavier than the hogback or fly creek option, and you can stand up in them - something I find a bit more valuable when jamming 4 people into a tent rather than just two. Once you have to step around or get overtop of a person, this just makes it a bit easier to deal with.
Great suggestions, thanks. I went around-and-around among the tents. I like the height of the Sangra-La. I own a tarptent and like that a lot. After reading replies to the following thread, among them Henry Shires - admittedly biased, I think I prefer the Hogback.
"so consider the dimensions carefully lest you get something so big it can't be pitched anywhere." Yes good point. The Hogback (51 sq feet ) takes a bit less space than the Shangri La3 (59sq feet) , the SL5 takes up 90sq feet (in case you are wondering, all of the floor space inside the Hogback is usable right up to the side walls)
The blue mat is 6' long, the Hogback floor is square. franco@tarptent
I actually hadn't considered the square hogback floor - I've slept a number of nights in it's little brother (the scarp 2) which is very nice and roomy, but you lay lengthwise with a door next to each side - with the hogback you'd be able to orient the other way so you can all get to a door without stepping over each other.. that's not too bad with a couple kids.
I'd still try rigging a couple two person tents as "rooms" with a tarp "common area" in between