I'm not advocating for corporal punishment. I haven't seen where it's terribly effective unless it is deeply embedded in a culture. But I guarantee you will use force if your son goes after your daughter in a physical dispute. There simply times when it's necessary. Hopefully, your strategy will work most of the time, but there will be a power imbalance no matter what you do until he is the same size as you. And then the nature of the imbalance will shift from physical to something else until your child can stand entirely on his own as an adult.