Girls, hi. How do you react to other children who try to hurt your kid…

Girls, hi. How do you react to other children who try to hurt your kids? What do you do or say? The husband's nephew (5 years old) constantly pinched his daughter's bottom while she crawled around, constantly pulling her, pinching her, hitting her on the head. When you scold him, he denies everything, even if he knows I saw it, the mother-in-law makes a remark when she sees it, the ashy girl pretends not to notice, in plain sight, nothing is visible. And recently, there was such a case, I was eating with my daughter on the bus. A woman with two children came in, took her son in her arms, her daughter was about 7-8