"Child abuse"? No.
"Child abuse" is abusing children either with violence, sexual interference, neglect or active psychological trauma AFAIU.
When a child reaches an age where they're able to notice that there are two (main) physical differences and asks questions, balanced unprejudiced answers should be given IMO. There is nothing wrong with keeping schtum in the interim.
So there's my tuppence worth, disregarding the preordained nature of your question.