What a great idea, and I fully recommend doing this!!!
My dad built a workbench for me and my brother when we were little. He put a drawer on each end, one for each of us, and made two shelves on it, again one for each of us. The shelves had holes drilled into the front part of them to put a hammer, screw driver, pair of pliers, adjustable wrench, and even a pencil.
What I remember most, and what I still have to this day, are the two small/mini vise's he put on the bench, one for each of us.
I have my mini vise. It is all metal, and looks exactly like a big vise. It even works just like a big vise, as it can turn 360 degrees around, and the jaws also rotate around.
It doesn't matter what you and the grand kids actually make on this workbench, even if it is only nailing a few boards together and painting them. What you are really making are memories that will last you and the grand kids the rest of your lives!!!!
Jim