“What happens if the custodial mother relocates the children without telling the father? Doesn’t she have to keep him aware of the childrens whereabouts? We have visitation, but now have no idea where the children are. All we know is that she moved away with them 2 days ago and there’s no forwarding address.”

For one, you can file parental kidnapping charges with the police. Additionally, each missed visit can be a count of contempt which can bring with it a fine and/or jail time. Keep track of what’s going on in a journal.

Some orders are specific about notices about relocation, and others are not. If your order is specific, then that would be something the police could help you out with. If it is not specific, then there may be less that the poice can do for you.

Does she have any other family you can contact to find out her whereabouts? Friends? Send mail to last known address and it may get forwarded. E-mail? Private Investigator? Skip trace?

If you never miss a visit, there is no specific order concerning update on residence address and she is just keeping her changed home residence secret from you, she may not have even done anything illegal.