"He received a message from this girl."
Translation:On dostał wiadomość od tej dziewczyny.
That would be a very specific reason not to accept "dziewczynki"... While it's true that "dziewczynka" is younger, we still accept "dziewczynki" here, it should have worked. But if you made a typo somewhere, then you probably got corrected to the main answer, which uses "dziewczyny".