"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.
no child is 'pas d'enfant' in French.
The child (English) -> l'enfant (French)
Selena Gomez n'est qu'une enfant
un enfant
"mon enfant" for a male child. "mon enfante" for a female child.
You can say "seulement moi" in French, which means "only me."
bel enfant
You would say "un enfant solitaire" in French.
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).
appréciez mon Enfant
Fat Child - Graisse enfant
Un jeu d'enfant