I agree. It is also not appropriate to use for adult victims of trafficking for sexual exploitation. Sadly the term 'forced into prostitution' is commonly used, even by support agencies, govt departments and media.
I think that there is a general squeamishness about saying "this woman was raped several times a day by a sequence of strange men", and instead it is described a "forced to have sex with several clients a day".
It comes from the same mindset as victim blaming for victims of rape, and means that the situation is described from the viewpoint of the man - "I thought she was a free-willed prostitute, I didn't know I was raping her", rather than describing accurately what has happened to the woman.
It is also a way of making the woman or child 'other'. By calling them a prostitute, with the negative connotations it holds, the victim is seen as having done something to have brought the situation upon themselves, and to have behaved in a way that our sisters, mothers, daughters and ourselves never would.