Always, where children are concerned, use double guards. Our Petey, who is aggressive toward strangers, used to be either in the crate with his muzzle on, or in the crate behind a locked door. The double guards assure no mistakes will be made and he will not be able to practice the bad behavior while you rule out medical issues and re-train him.
Petey has become so accustomed to his basket muzzle that he visibly relaxes when it is put on. He knows he no longer has to be on guard, and it relaxes us, so it helps him relax.
Kids don't listen, so we learned the hard way that in the crate alone was not sufficient. We had someone open Petey's crate despite Petey's warning growls and receive a bad bite on the hand. We also had a child open the crate while he had a muzzle on and receive a muzzle punch to the face. Without the muzzle that would have been a facial bite.
Between Dr. John Cirribassi and Laura Monaco Torelli, we have gotten Petey to the point that he can be in a separate room, behind an x-pen, receiving treats for looking at the strangers, and can come out and socialize once everyone is settled down.
Hope this helps,
Nicole