It is developmentally normal for small children to struggle with impulse control and to hit, pinch, push, grab etc.
But the experience of being hit, pinched, pushed, grab etc is painful at any stage of development.
When another child is doing these things to your child is can be incredibly triggering and hard to navigate (esp if the other parent is not intervening and is a friend or acquaintance).
Remember that these are moments to teach your children 2 things:
1. Body ownership and permission to set boundaries with people who hurt them
2. That you care about their pain and will calmly but firmly protect them from people that are using hurtful hands or words