I think المدلّلة means spoiled.

حسب فهمي للجملة، فإنّ مصطلح baby girl يعني طفلة يحبّها أحد والديها كثيرا، و هذا ما جعلني أترجمها بعبارة "مدلّلة."

I'm not even sure it refers to a daughter.

baby girl
Someting a boy calls his girl wen he is in love. most of the tym the girl is quite, shy (sometimes), cute, sexy and everything a boy could want.
http://www.urbandictionary.com/...rm=baby%20girl

Even in case of a daughter, I didn't find any proof this means the only girl. I'm not sure it should be translated as مدلّلة

http://onlineslangdictionary.co...on-of/babygirl

I sometimes have my head in the clouds when I translate (in fact, this happens to all translators). When I translated it, I was thinking about her as "daddy's little girl" and in this case, the word "مدلّلة" might fit perfectly. As for this sentence, it literally means that she's his infant daughter.

Could you please translate this sentence into Russian?

Thanks for your explanation. I have one more question. According to my dictionary, الصّبيّة means a girl. So it's basically the same as فتاة صغيرة? What else could be used in this sentence? بنت, شبّة, مراهقة, صبيّة

The word صبي
also means baby (infant). None of the other words you gave in your last message fits the meaning of baby girl other than صبّية.
شابّة is a young woman

مراهقة is a teenage girl

فتاة is girl, and it can mean a young girl, a teenage girl or a young woman

فتاة صغيرة is a little girl, but if we use it in this sentence, we wouldn't be conveying the meaning of her infant age.

بنت means daughter

Thank you. Very informative.