There are numerous synonyms for the phrase "bonnie young girl" that can be used in various contexts. Some of the commonly used synonyms include "pretty maiden," "charming lass," "lovely damsel," "adorable miss," "beautiful debutante," "delightful belle," "enchanting young lady," "exquisite mademoiselle," "graceful nymph," and "winsome princess." Each of these synonyms brings a different nuance to the original phrase, but they all describe a young girl who is attractive, charming, and graceful. These synonyms can be used in literature, poetry, or any other form of creative writing where a writer wants to evoke the image of a beautiful and captivating young girl.