What should I do if I was not notified of a meeting as mandated by my child custody order? How?

Without seeing the order, yes he should have told you about the meeting. However you also have an obligation as a parent to be in contact with the school and be involved.

There is probably no action you can take against the Countybut you might want to be certain to remind them that you are entitled to notice of all such meetings. As to your former husband, if he has violated a court order you can try to invoke the court's contempt power. But judges who sit in family court hear far too many complaints which may not be all that serious. So it could just be seen as an attempt at revenge. Why not just discuss it with your ex? Or if you two can't talk calmly, send him a polite but firm letter.

File a contempt in family court against your ex.

It is doubtful you have any cause of action against the county, but you certainly have a right to complain about the fact that you were not informed about the meeting and given an opportunity to attend it with the school. You can make a complaint in a motion before the court, and you can The school to provide you an opportunity to meet with the person who set up the individual educational plan.