He's a dick, but yes, you should pack your children a change of clothes. God, how humiliating for a child, especially a preteen girl! In my opinion, what he is doing is child abuse. You may even want to keep a change of clothes at school, just in case he decides to be a real jerk even when you send them to him with extra clothes. The children should not have to suffer because of his abuse. Neither should you, but better you than them, right?