"The little girl wants to drink water."
Translation:Bé gái muốn uống nước.
in English, both can be translated as a girl. however, to be more specific, "bé gái" usually refers to a much younger girl (from a baby girl up to an elementary schooler at most, if not around a 6-7 yr-old girl). "con gái" is more generic for girl (more likely being used to specify the gender, as opposed to a boy) and, like you mentioned, can also refer to one's daughter (no matter how old she could be).
ps. girlfriend is usually "bạn gái" or the gender-neutral "bồ".