Shared driveway, yes, it is not unreasonable to ask them to sweep the piles off the road, at some reasonable frequency, but with the caveat that if they were there first, or this is a rural area, get real, it is part of what you bought into.
On the other hand, really? I can think of a whole lot of other things to be concerned about.
If these are young kids learning to ride, why not try to have some compassion, as the kids are trying to juggle four balls, while holding a spoon on their nose, and whistling Dixie? This feels like someone complaining that their kid isn't changing their own diaper. If it bugs you, get a manure fork and walk the driveway; you will get some fresh air and exercise, and do something good for you and your neighbors. The kids might even thank you, and wouldn't that be a nice outcome?
I would try to take some joy in the fact that the kids are probably having a wonderful, joyous childhood experience, and might just turn into a happy well adjusted adults, in your town, doing happy well adjusted things for your community.
All the best,
Peter