Hi A., I have a small exercise trampoline in my livingroom (no other place to put it) but you can purchase inexpensive ones that the kids could play on. Be sure you get one that you can buy a handle that attaches so they have something to hold on to.
The reason i suggest this is that when my 3 little grandboys, 7, 4 ans 2 come over they are constantly on it. Great exercise and it wears them out!!
Another thing would be to do exercises. They might have a dvd for kids to do arobic type exercises. jumping jacks, running in place, or if it is agreeable to you, run in a certain pattern in the house - like thru the living room, into the kitchen, and back into the livingroom.
Good luck. It is going to be a long winter. M.