Since mine was referenced, I will chime in with my experience with small kids. (My kids are 5 and 2)
You say they don't need much space, but mine use quite a bit of space. They are typically excited while camping, so they may not take up much space physically. But by the time you account for their movement, they take up a lot of space.
The slide out bed that was mentioned, originally I was hoping to have both of my kids sleep there. It should be wide enough if they sleep perfectly still.
Obviously that does not happen, so that 5 years old sleeps there and 2 year old sleeps with us on the main floor. It is working for now.
Another design decision turned out to be a good decision was to have a extra headroom. It is high enough for us to sit and move around comfortably.
There were days and nights that were too cold or too wet and we were forced to spend extended time inside. We are able to eat and play inside without being on top of each other.
Not the best picture, but you get what I'm talking about.

I posted a lot of pictures on my build thread and have bunch more if you need specific area.
