My ex partner and I have 50/50 custody. Her new partner assaulted her very seriously and was jailed for 14 months. He has come out of jail and they are now living together again. I am scared for the safety of my children when they are residing at her place for 50% of the time. Is there anything I can do to protect my children. I obviously don't want to wait until something happens.

File for a change in your custody arrangement based on the behavior of her roommate. Did you have a lawyer for your custody agreement?

You will need to get your lawyer to request a change in visits based on this. And link actions or dangers to the children to their behavior.

If it were me I would file for an ex parte order right away and get things changed as soon as possible.

https://info.legalzoom.com/ex-parte-custody-order-25626.html