I am not a fan of kids on boxes for kids. I made several large boxes for my kids. They were 12 x 24 x 16" tall with carpet casters on them and a thick, long rope handle on one end. They were made from 3/4" pine with dovetailed corners and heavy duty bottom. We had the kids pick up toys and put them in at bed time. The rolling boxes were often used for pulling each other around, as a fort, hiding place and many other things. They all survived my three kids.
It is interesting to note that a large cardboard box is often a kids favorite toy.