If you're going to make a workbench designed for kids, a neat design feature would be to make it possible to grow with the kids. Maybe some nesting tubes for legs would allow it to change in height over time.

Apart from that, look at the surface of the bench. Whether you want to use wood, laminated particle board, or lego base plates, making the surface extendable can help small arms reach the back, then expand horizontally as they need more bench space.