If my daughter's mom and I have a custody agreement, what happens if she breaks that agreement?

There is not nearly enough background information to answer that question.  What kind of agreement -- oral?  Written?  Implicitly understood?  Court-ordered?  What kind of "break" (disappeared from town?  10 minutes late on visitation?)  What is the relationship between the parents? (married?  Divorced?  Never married?)  Is there any kind of legal structure in place? For background, see http://willicklawgroup.com/child-custody-and-visitation/.  But you are going to need this question -- with lots of further information -- past a family law specialist in order to get any kind of meaningful answer to it.