Oh geez, this is a HUGE topic in my weekly tele-seminars with my other moms! Here is what I tell them, you can speak to the parents, but will it make a difference? In most cases, no. The parent will take offense or they will feel you are attacking them or saying something "bad" about their child. It really depends on the parent. Typically, children behave as they see and are taught, so consider that before speaking to the parent.
Here is the best thing to do. Talk to your child. If the other child is doing or saying really bad things or is harming your child, then yes, you will need to take action, appropriately, to handle it right away.
If the other child behaves in a way you just don't agree with, then you have to turn to your child. What this means, is that you need to talk to your child to explain that the way the other child behaves is not nice and its not okay. Its important to choose wise words.

Even though we might want to say something else!
Explain to your child that the other childs behavior is not okay and will not be accepted in your home. This way its very clear to your child that, that behavior is not good and should not be followed. I always tell my children, I better not see that word or that action used in this home and I better not get any phone calls that you were saying or behaving that way out of this home. That is the first step.
The next thing to do is explain to your child that he/she does not need to accept the behavior from the other child and needs to speak up and defend him/herself appropriately. You can give your child options depending on each situation.
Good luck to you!
Coach Ann
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