As in other exercises, the particle for plural is not always needed. Sometimes is a bit confusing, I know. In this case in particular there is a number indicating it is more than one girl, so apparently you can drop the "plural particle". In Korean they tend to drop even the subject... so to drop plural particle is not a big deal hehe Hope I could help.
First youre missing a counter for people, which is 명 Second the sentence structure is the incorrect order, the number of objects should be placed between the subject and verb. Someone would understand this sentence but it isn't grammatical, like "There are little girls eight"