We don't know enough about the situation and what your custody order states. If this violates your visitation or custody rights, my understanding is that you can:
- have an attorney write a letter informing the mom that you will take legal action and see if that works.
- file contempt of court motion (you can ask for your costs and attorneys fees too.)
- call the police.
This isn't my area of specialty but it's what I understand the options are.