Having them bop each other for a bit beforehand is a very good idea- kids are sensory creatures and they’ll understand that it’s Not Fun to be hit hard once they’ve been hit themselves.
I’d also suggest restricting safe to hit zones based on age, say anyone under 10 would be legs and arms only, over 10 would be allowed to take torso and/or back shots too. They’ve got a bit more finesse when it comes to fine and gross motor skills by that age so hopefully they won’t take too many head and sensitive anatomy shots.