"I am a child."
Translation:Je suis un enfant.
It is indeed one of the few French words that can be either masculine or feminine, depending on the sex of the child (by the way, child can also be a boy or a girl).
There are a few others, often to address the feminization of certain jobs, like "un capitaine" or "une capitaine".
But remember that fruits have no sex and that masculine and feminine genders are not sexual qualifiers.
No because there is a rule:
il/elle est + modified noun (= determiner + noun) changes to "c'est + modified noun"
- he is a boy and she is a girl = c'est un garçon et c'est une fille
The same rule applies to plural pronouns ils/elles + modified noun = "ce sont + modified noun"
- they are children = ce sont des enfants