My ex has moved and won't tell me where he lives.

You've left much quite a bit of pertinent information out, so fmy answer assuems: 1- The children are with you currently; 2- Your current child custody orders are silent on the subject (be sure to check for any supplemental orders issued by the court at the time of your divorce decree or other custody orders.  Also if your current custody orders were the result of a chilld custody modification, check to see whether they include a provision stating that any original custody orders that aren't inconsistent with the new orders remain in effect, or words to that effect.  If so, check all original orders for any requirement that a parent keep the other informed of all change of addresses.) 3- Your former spouse has moved WITHIN the same city/town where you all live. Of course, you could hire a private investigator to help locate him.  If he's moved within your city/town, it shouldn't be that hard to locate his residence. You might also contact local law enforcement , explain the situation, and ask them to be present at the next parenting exchange so that you can ask him iwhere he lives n their presence, since there's at least a potential threat of harm to the children, depending upon the condition of his new residence. failing those options, if you withhold the children from him, you risk a contempt of court charge, with it's attendant penalties (possibly jail and fines).  While you might not be found guilty of contempt, you would still end up in Court, having to defend your actions.  You should really seek advice from a family law attorney in your area. It's possible you could move for emergency orders ( which can be issued quickly IF the court finds the children are at risk, but it's not an easy thing to accomplish without the help of an attorney.  Good luck!