I had a similar problem with my RB Meezer Ming.

He always loved everybody, until one day, a friend of mine was visiting with her young daughter. It was a hot summer day, and the air conditioning was down for repair. The little girl kept pestering Ming and being a royal pain in the a$$. I told her to stop, that he didn't want to be bothered and he wasn't feeling well because it was hot, but she kept up. Finally Ming turned on her and bit her before running off.

After that, he would not tolerate kids (other than my own), and would actually scream at them and go into the attack mode, if they came in to "his" house. He never acted that way with any of the family, or other adults tho.