I make my legs 32 inches but the height is a personal preference since your son will be viewing it also.very crud drawing in scarm but it gives an idea of the wood I left the table top up so you can see the wood structure under.I make my second level at 7.75 so on top of the base table top I put 1x4 legs cut to 7.25 then on top of the legs I put 1/2 MDF for the base for the second level .