My ex partner won't give me my children back... He has had them for 5 months now, He threatens me when I attempt to take them with me.. What do I do ?

First, ANY question on law needs to include your general locale as laws vary by area. Second, we need more details. Is he on the birth certificate for both? How old are the children? Are there any court orders for custody or visitation?

Why didn't you just go with a police officer to get them back?

After 5 months, you will probably need to go to court to establish custody and he may very well win custody.

Agree, is the person their legal parent ? Who has legal custody from the court ?