Perhaps they might be persuaded to do other things, whose effects they can see more directly.

Also, encourage children in their activism. They are good at making complacent parents take notice at least, if only for social signalling :-) and hopefully some of it sticks when they grow up.

Voting shouldn't be seen as the basic thing, which people need to do to show they care in the world. Many people see voting as mostly pointless, and they might even be right compared with other things they can do.