My father built a simple child-size workbench out of scraps for my children when they were small. Now my children are grown and my grandchildren now use it when they come to visit. The bench is nothing fancy, but I think it is really cool to see the little girls work on a bench built by their great-grandfather.
Top is 1/4" hardboard over 1/2" OSB, width 48", depth 24" and height 29". It is equipped with two, small, old-style woodworking vises. It carries a lot of scars, splashes and dribbles from years of kid use, but that all adds to its character.
Bench in action!! Before Thanksgiving, I cut pieces for two bluebird houses and pre-drilled all the holes. The two girls (5-1/2 and 3-1/2 ... yes, the "1/2" is very important in their view!) glued, nailed, and painted ... with assistance from my wife and me. They worked hard and stayed focused as can be seen in their faces ... when concentrating the younger one frowns and the older one bites her tongue.
I've since picked up a pair of 10 oz. claw hammers and child-size safety glasses for next time.