I just told my kids and now my grandchildren that on Shabbos we have special things that we eat, drink, do, and play with, that we don’t do when it is not Shabbos. I keep special Shabbos toys for the little ones, and there are treats they are ONLY allowed to have on Shabbos,a nd they look forward to Shabbos for that reason alone. I explained muktzah to them as things that we don’t use because then Shabbos would be just like any other day, and because it is special to us we also act in a special way.
My children were never permitted to use riding toys on Shabbos, though I see the frummest kollel families allowing their kids on tricycles and other such toys. I don’t happen to think that’s a good precedent from a chinuch standpoint alone.