I have a 210 Popular with the front bed option. It is a flat folding bed surface with about 3" of foam padding. When laid out on the seats is is quite level and the wood backing under the foam mattress keeps it that way. On mine at least I think someone who was no taller than about 5'8" would be OK, but that's pretty much true for the twin beds in the back of my unit as well.
Depending on the age of the kids it might be more fun for them to carry a tent and let them sleep in it outside. Probably be better for the parents too. LOL