A problem with coaster brakes is that shoelaces can get caught. Unlike a handbrake, one cannot turn the pedals backwards to release the laces. I had several crashes as a child due to this, so perhaps kids' shoes should have their laces taped or otherwise attached to avoid such a problem.
Hand brakes do take some getting used to, but I doubt it's anything a kids can't learn within a day in terms of normal use. Training for emergency stops would take a bit more work.