Enfant is one of these few words called épicenes, i.e. which has the same form for both genders, is masculine and feminine.

Here we know the child is a little girl so she is une enfant.

What might be even more confusing is despite being used in its feminine form, the masculine possessive son is used but this is for euphonious reasons.

The fact is sa enfant is impossible, like sa amie becomes son amie.