Robin, here's how I would do it. Buy a sheet of 1/4" AC or BC plywood at the home center and have them cut it into one piece 4' x 4' and 2 pieces 2' x 4'. Also get enough 1 x3" (3/4 x 2-1/2" actual) to frame the outside of the plywood. They probably will not cut that for you. Cut the 1 x 3 so the long pieces are on the 4' sides and using 6 x 3/4" wood screws, attach the long sides. Cut 4 pieces for the short sides (approx.19" long for the 2' sides but measure and cut actual dimension) and 2 pieces for the remaining 4' sides (approx 43") and attach them the same way. Then drill and run 2 3-1/2" deck or drywall screws at each corner thru the 2-1/2" dimension and into the butt end of the 1 x 3. Apply a coat of shellac to all 3 panels, sand and paint to suit with non-toxic paint. Finally, apply the piano hinges and hooks and short lengths of chain to the top so the assembly cannot unfold to flat so it can easily be tipped over.
I cannot control construction or usage, so be careful in both. Liability could become an issue. As I'm sure you know, kids can do unexpected things so keep this in mind. Henry