Parents, your child is at a park or playland. Do you let them play with everyone that is there or tell them not to play with certain kids?

I have a four year old son. I have noticed many times while at parks and playlands that some kids are just rude and inconsiderate of others. Some kids will not play with certain kids. I have watched kids ask others to play with them and the child that they have asked just walks away. Is this something that parents have control over?