Depending on how young the girl is you would be more informal with her. You address her with "du" and say "Hallo" instead of "Guten Tag".
Allemand (german boy) Allemande (german girl)
Since in German young and boy have nearly the same meaning, you can just say "Junge" and already someone would know that it's a young boy with saying young.
My girl - mein Mädchen.
"Girl" translates as "Mädchen" Madchen
Pfadfinderinnen
weibisches Mädel
My Little girl = Mein kleines Mädchen
Junger HundWelpe (puppy)
I love that girl = ich liebe das Mädchen
Girl is "chica" in Spanish. A very young girl would be "niña" (which is a baby girl). A young lady (unmarried) is a "señorita."
ein kleines Mädchen