Is a father sending his children with their mother to a Muslim country, out of fear for them from deviance in a non-Muslim country, considered disobedience to his mother who opposes that?

You are not considered disobedient to your mother by disagreeing with her advice if there is a weightier religious benefit in doing so. You should be gentle with her and try to convince her, as your children are your responsibility, and your mother will not be held accountable for you if their religion and morals are lost due to following her advice.