If you haven't discovered this yet, you might look at artist's foam board, which is used by architects and the like for building mock-ups of buildings. Its a layer foam sandwiched between two pieces of cardboard. This makes for lightweight "skin" for the missile launcher boxes, and you can cut it into the circles for mounting the PVC launch tubes.
As for supporting the weight of your son, I would suggest you solve this problem first before getting too much farther along in your design phase. If you can't solve that problem, then the rest of it won't matter.