In Serbian there are affectionate diminutives only for some adjectives like: lep-lepuškast, ružan-ružnjikav, some colours crven-crvenkast, plav-plavičast, žut-žućkast. For the adjective pametan I can not remember such a diminutive. To a child you can say that he/she is MALI PAMETNJAKOVIĆ (which is a noun) or you can say PAMETNA GLAVA/GLAVICA, if you are talking to him/her this would be said in the vocative PAMETNA GLAVO/GLAVICE.

This discussion is very helpful and it shows that sometimes even the native speakers have doubts. I'd say that words "pametnjaković" and "pametnjakovićka" have negative connotation whereas "pametnica" and "mudrica" have more positive meaning. There is another word that we forgot about. It has no positive meaning in colloquial usage - "štreber" (German origin). "Smarty" may also have negative meaning - know-it-all, wise guy, smart aleck... I might be too much into details, but it might be helpful. "Štreber" represents a person who studies constantly, usually has low social skills, not interfering with others... or just studies more than his counterparts. All in all, if you want to give a compliment, use Kamenko's suggestion.

We use PAMETNICA or MUDRICA for such kind of person in colloquial language.

Mislim da je odgovor pametniji.. Tako bih ja rekla za neko dijete.

Как уже Каменко сказал тебе, мы скажем: Паметница или Мудрица. Если бы ты хотела быть саркастическая, тогда будешь сказать: Паметњаковић/Паметњаковићка