Question

A doll of the girls. (Нужно сказать не кукла девочек, а девочкина кукла) Склоняюсь к выбору "Girl's doll", но по идее это ед. число, а не множ.

Answer 1

Girl’s doll. Запятая наверху показывает принадлежность. «Деточкина кукла».

Answer 2

"Girl's doll" is the correct choice because it indicates that the doll belongs to a girl. Even though it is in the singular form, it implies that it is a doll for girls in general. If you want to indicate that there are multiple girls who own the doll, you could say "The girls' doll" to show possession by multiple girls. But in the context of a doll meant for girls in general, "Girl's doll" is the appropriate choice.