2 Answers

For what legal jurisdiction(s)? Is it a legal separation? What type of custody..legal, physical? History of neglect, violence? Support issue(s)?

As you can see from the answer Clonge gave you, there are many variables.  Generally, possession is 9/10 of the law, and, without court interjection, that's probably the way it goes when parents separate.

If you've a concern (which, regarding children, there always SHOULD be), get to a lawyer or legal aid office and get something solid with a judge's stamp.  My son had an agreement that hadn't been finalized, and when the mom didn't have all 3 of the children where they should have been when they should have been there, he had to go looking for the missing one, kept them, and has had them ever since (now legally, full custody, she with supervised weekly visit).  Long story, but you get the picture.