Albert Barnes Commentary Matthew 12:27

"And if I by Beelzebub cast out demons, by whom do your sons cast them out? therefore shall they be your judges." — Matthew 12:27 (ASV)

By whom do your children, etc. Christ was not satisfied with showing them the intrinsic absurdity of their argument. He showed them that it could just as well be applied to them as to him. Your disciples, taught by you, and encouraged by you, pretend to cast out devils. If your argument is true that a man who casts out devils must be in league with the devil, then your disciples have also made a covenant with him. You must therefore either give up this argument, or admit that the working of miracles is proof of God's assistance.

The words of Christ, here, do not prove that they actually had the power of casting out devils, but only that they claimed it, and practiced magic or trickery. See Acts 19:13.

Your children. Your disciples or followers. (See Barnes on Matthew 1:1).

They shall be your judges. They shall condemn you and your argument. They are conclusive witnesses against the force of your reasoning.