I thought "dziewczynka" was for "little girl" and dziewczyna" for "girl" but here "little girl" is wrong. Have I misunderstood something ?
In this sentence it's genitive singular.
It would be "Nie znamy tamtych dziewczynek" (see the link in the comment before yours)
Is there an explanation on when is genetive used ? There doesn't seem to be any notes available on this skill.