فتيات literally means girls.
فتيات صغيرات literally means young/little girls.
On its own, فتيات refers to girls in general. Like the English word girls, the Arabic word فتيات can be used in reference to female children, adolescents, and young adults. On the other hand, فتيات صغيرات is more specific as it implies that the girls are little. I'd say under the age of 13 or so.