Do not bad-mouth your ex’s family. That is your children’s family, and they will likely love them and need that connection
Criticizing your ex’s family in front of your children can be just as bad as speaking badly about your ex. Studies show that children do not like it when one parent talks badly about the other parent. It makes kids feel bad and since it does, to act out in such a way is just plain selfish. The same goes for speaking ill of your former in-laws and other family members.
In your mind, you may feel justified to hold animosity towards your ex’s family. You may blame them for contributing to the break-up or for acting in ways that hurt you. But whatever enmity you hold with those people, you need to remember that the conflict is between you and them and NOT the children and them. It may not be your extended family any longer, but they are still grandparents, uncles, aunts and cousins to your children. Your kids will likely want to continue fostering those relationships and may even need some of the support they can provide. Your anger could tempt you to block these people from your children’s lives, but to do so would not only punish your ex’s family but cause harm to your own flesh and blood. It is better for the children to allow relatives to attend birthday parties, graduations, school concerts, and sporting events. You need to take the high road and consider what is in the best interest of your children.
Naturally, any parent has an obligation to protect their children and should keep them from any actual danger such as people who suffer a drug addiction or participate in criminal activity. But if no threat exists, you should cooperate and allow the relationships to continue. To do so goes a long way in showing that you are a loving parent who is above the frays of family discord.