Question about my son and daughter?

Lol that's a hard one to answer I would say its fair play but as a mother I would also lean towards the protection part, If you do give permission make sure she knows that "hard" can effect him as an adult and set the ground rules after wards "This is why I specifically asked you to stop so it did not escalate to this" and if I catch either of you doing it again there will be serious consequences.