Legal Question in Criminal Law in California

He would have to apply for a Visa. There would seem to be little likelihood of a Visa being granted to him. He best approach would probably be to retain an attorney in the community in which the children live to request custody. No information regarding the present circumstances of the children has been presented. He may have a very difficult time obtaining custody.

NO, and even if he hired an attorney to do so and represent him in court, he will have the problem of not being able to come to court when necessary to testify, and the problem of not having an in-state US residence where the court could order the children to reside, and the problem of not being a fit parent because of the crime[s]. Deportation means he has no rights to be here, and he doesn't get to come back. That's the whole point, and the consequence of being illegally here.