Mrs. Trotter warned Gilly not to be mean to William Ernest because he was smaller, shy, and appeared more fragile than Gilly. He was younger than Gilly, and she could be both sharp spoken and aggressive. Gilly insults the TV show William Ernest is watching, and Trotter thinks she is calling W.E. rretarded. Mrs. Trotter told Gilly she would not tolerate anyone making William Ernest's life being any rougher than it had already been. She told Gilly that as long as he lived under her roof that no one was ever going to make William's life tough again.
She would eventually support Gilly in the same way, but Gilly doesn't realize this at the time. Gilly is mean and hateful most of the time. She is very "rough around the edges," and Mrs. Trotter isn't going to put up with that personality hurting William.
"Trotter says, 'He's had a rough time of it in this world, but he's with Trotter now, as long as the Lord leaves him in this house, ain't anybody on earth gonna hurt him. In any way.'" Pg 12.