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 will not intervene. Wait and see how your child handles the situation by himself. I will also suggest speaking to your child after the episode to find out what was his thought process. If he was willing to share, commend him for being kind and nice. If he reacted negatively, asked him why he did that and ask him how he could better handle it if the same situation happened. Think this is a great learning opportunity for him. Just my two cents. :)

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).