From Longman Dictionary of Contemporary Englishgood girl/boy/dog etcgood girl/boy/dog etcused to tell a child or animal that they have behaved well or done something wellSit! Good dog. →good
Examples from the Corpus
good girl/boy/dog etc• I am Pa's best boy.• Randolph worked his hardest, pulling away, while Santa delivered all the presents to the good boys and girls.• He's a good boy, and he's very strong.• I tried to be a good girl and stay out of the way.• He coughed, told Oliver to dry his eyes and be a good boy, and walked on with him in silence.• This was where good boys came after they got killed by Rupert.• Good boys, good boys, good boys.• He had been a very good boy indeed.