I'm thinking aloud here . . .
One would normally immediately think of a metal framework but I don't think that would be a good idea for a 7 year old in action.
I would baulk at doing this because it is likely to be a lot of work for a brief outcome.  However, since you are a wonderful aunt, I'll make a suggestion.
If you get some very large pieces of cardboard (or stick smaller pieces together to make a large piece) and cut circles about two thirds the size of the total diameter you want.  You'll need at least four.
Intersect them in such a way that your niece can stand in the space between.  Take some strips of card and lay them to create the surface.  This won't be easy!
Then you get to the paper laminating bit, the easiest part of the process.

Still think it's a good idea?
Perhaps someone can think of a really sensible, simple way (like waving a magic wand???).
DavidO

ok I'll join in with the thinking out loud. You could try a framework of willow canes or something similar then cover with several layers of paper or cloth.
Sue