I made 3 "Cube-Seat" for my 2 kids. Really nice , beause u can switch them, in 3 Position's. 2 Position's as Seat in different heights and 1 as table. It is really simple to make. And your children got much fun with it!
So let's start !
The cube is 400mm x 400mm x 400mm . Very simple ;)
So you need :
2 Sides with 400mm x 400mm
1 Rear wall with 400mm x 364mm
1 Seat panel with 364mm x 382mm
I don't use any varnish or paint , i think it's not necessary for a children furniture. Your kids will colour it later ;)