"Paradise Valley Unified School District requires children to wear hats when they go outdoors"
This isn't out of line--these folks are in the desert. We have a friend who just died at 43 of melanoma so I guess it touches a nerve.
But let them run and play, for goodness sakes.
Provide plenty of covered play areas and most kids will choose to spend a lot of time in them. Much more effective than hats, and much less intrusive for the kids.