It says "loc." for dobrym but "instr." for dzieckiem. Shouldn't the adjective here be in the same case as the noun it describes?
The adjectives have the same form in Locative and Instrumental and yes, both adjective and the noun are used in Instrumental.
Good, but such a question/answer set really calls for specifying the subject: Ty jesteś dobrym dzieckiem!
After all, it's very likely to be even emphasized by voice :)
Maybe when you refer directly to a child, you could even say "you are a good boy / girl" insted of "you're a good child" ;)