I made two of these step stool / chairs for my daughters when they were small. They were a big hit, besides being a stool and a seat, they were a incorporated in a lot of play time. I probably have 6 or so of them out there as gifts to other kids, and they've been well used. I've always just used a pine 1 x 12, any other woods would work, pine is light weight enough for little ones to move around. They're basically 12" or 12-3/4" cubes, the half blind dovetails dress it up a little, but a dadoed joint between the top and the sides holds up well.