What do you do if you see a kid trying to steal your kid's toy (the one he's playing with) or tries to hassle him out of, for example a toy car so he can get in it? Do you intervene or wait and see how your child reacts to the situation.

I would not jump in immediately, would probably wait a while and observe how my child reacts to the situation. I believe this would help with the development of their social skills, their problem-solving skills and their independence. But I would immediately jump in if it gets out of hand and they start getting physical (hitting, biting).