What do I do if I have joint custody and the other parent commits crimes while my child is in his custody?

If the other parent has committed a robbery while the child was in his custody, and you have evidence of that, you owe it to yourself and to your child to file for modification of custody as soon as possible, since the other parent is placing him in grave danger.  You may also want to consider filing an ex parte motion for emergency change of custody.  You can file this motion on your own if you understand the law and know what you are doing, or you can speak with a lawyer in your area to get more advice and to possibly have them handle this for you.   Whichever, you do decide to do, you should do it as soon as possible.