Can I press criminal charges against someone causing intentional pain against my child

This is a real grey issue. From your facts, it's unclear whether your daughter suffers any injury or headaches as the result of the wife's actions. You can put your concerns in writing to your ex and tell him to notify her to cease and desist the behavior. Then you have proof he was notified of the concerning behavior. If she doesn't stop, I would have a school counselor speak to your daughter and see if it rises to the level of abuse. The school counselor is a mandated reporter and has to contact Child Protection Services. You then have the option to file a family offense petition against his wife in Family Court or move to modify your ex's access requesting that the wife not be present during his visitation. Without proof of injury, harassment or emotional harm, the Court may not grant your request for relief. Find out if she's doing anything else. If your ex knows what she's doing and it is considered abusive, he could be charged with failure to protect your daughter, or complicit in the abuse. You need to get more evidence and contact an attorney who is familiar with domestic violence law.

As a criminal law matter, this is likely not enough to support assault charges, unless there is some evidence that the child is being intentionally injured (in this case, suffering "substantial pain"). It certainly is something that should be monitored. However, this type of behavior may be something which can be addressed in Family Court with respect to visitation and making sure that the child's "best interests" are being served. You should consult a family law attorney to determine what more may be done.

Have you spoken about this situation to your ex? Is he aware of how your daughter feels? Perhaps you can bring this to his attention and ask him to protect your daughter or explain to his wife that she is causing pain when she does your daughter's hair.