In French, you would say, "Elle est fille unique." This phrase directly translates to "She is the only child."
The child (English) -> l'enfant (French)
no child is 'pas d'enfant' in French.
Selena Gomez n'est qu'une enfant
"seul enfant, enfant unique". This is not gender specific, and the words "fils unique / fille unique" (only son / only daughter) are more common in spoken French.
un enfant
Only me is "Seulement moi" -Source = My main language is French..-
"mon enfant" for a male child. "mon enfante" for a female child.
bel enfant
Un efant solitaire
I am not an only child is "je ne suis pas enfant unique" in French. Although this is correct, the way we would say it is "je ne suis pas fils unique" (only son) or "fille unique" (only daughter). Or we would say "j'ai un frère / une soeur" (I have a brother / a sister).
Un jeu d'enfant
Fat Child - Graisse enfant