Hello girls! We live together with the girlfriend, until they buy an a…

Hello girls! We live together with the girlfriend, until they buy an apartment ? Such traditions. And we have children of the same age, my son is 1.3 and her daughter is 1.6. We used to play so well, sitting together. Now her daughter constantly runs up to my son as soon as she sees him and hits him, and I feel angry seeing this picture. Her husband and mother-in-law sit and laugh, and say "Oh, little boy's butt". This makes me even angrier. So, my question is, when do the kids learn to give back and not let themselves be hurt? As if we can't catch up to her because she's older...