There are some cities in the Europe where bus travel is free for all. The costs borne by the council and central government. You wouldn't see too many empty buses if we had that
The main advantage of your idea would be one of habit forming, (they might carry on doing this throughout life if they're used to it)
The main problem is that as children can't vote, there's little to gain from politicians bribing them with free stuff ... other than possibly it's for the...