The usage of "good boy/girl" in contexts such as you gave above are 100% idiomatic English. As such, a word for word Arabic translation would prove to be meaningless to a native Arabic speaker (or to a Spanish, French, Greek etc speaker).
"Good boy" in your sentence does not mean the opposite of "bad boy". It actually means "Congratulations!" Therefore the best translation for the phrase in Arabic would actually be: MABROOK aw MUMTAAZ or a similarly congratulatory expression.
PS...you could be a little more colloquial (Egyptian) and say, for example, "MABROOK YA WADD" or "BRAVO 3LEIK, YA RAGOOL" or their equivalents in Levantine which I'm not familiar with.
I should also mention - although it will sound weird to Arab sensibilities - that the expression "Good boy" (and sometimes "good girl") is very often used....you might even say it is MOST often used .....in American English..... to refer to the speaker's pet dog (!) more than to a human child. When the dog owner throws a stick or ball and the dog runs after it and catches it and returns it to the owner, the owner will most probably say "Good boy".
(Further expansion of this usage: When a dog owner call his dog, it is very common to hear him say: "C'meer, boy"...... Such a usage of WALED is, of course, totally alien to Arab ears and usage.)