If she does not return property to your Son or give you the keys to your car you have several options. If she refused to return them after you make demand she may be liable for conversion which can be a civil or criminal matter. You could also file an action for injunction to get the property back.
You might talk to local police about conversion and see if they can help. If not, go to small claims Court and ask them which action you can make in their Court to get these tings back.