妞儿 is very casual and distinctly Northern/Beijing dialect. It can sound endearing (for a little girl) or flirty (for a young woman), depending on context and tone. In formal settings, use 女孩 or 姑娘.
grammar
The 儿 is an erhua suffix - it merges with the preceding syllable and is not pronounced as a separate syllable. 妞儿 is one syllable: niūr.
