Anybody who gives an illegal water or food, can be charged with a crime. But they can go to court and claim it was the core of their religion, if they are Christian. Jesus says quite explicitly in Matthew 25, that we are to feed the hungry, etc. So this humanitarian assistance for illegals is the most Christian thing one can do. Jesus even said "You did this for the least of these my brothers, so you did it for me." I would say that's more important than Communion or any other ritual of the Christian church!
Trying to prosecute people for acting on their religious beliefs is doomed, and just might lose Trump some support among people who claim to be Christians.