There is no need for undershirts or camisoles, under a girl's tops, unless it is see through, or a white color which is sheer, or a thin t-shirt.
It is hot where I am. No one wears undershirts or camisoles, per girls/women, unless it is sheer or see through or a thin material in which you can see through or unless it is a 2-piece top with an under layer to it.
My daughter, since she was younger, I just would teach her, that "white" clothing is something that can be seen, through. Per her "breast" area. Especially if the fabric was thin. It is just common sense. Just like with white colored pants, too. One can see your underwear through it, when the fabric is white/thin, etc. So many girls and women, don't realize that. And then the whole world can see their underwear or whatever, through their clothing.
BUT if the clothing is not sheer or white or thin, then you don't have to wear any "bra" under it or camisole etc.
In the girl's department at Macy's, they have, for Pre-Teen/Teen girls, those training bra things, that simply looks like a "sports bra." That is what you mean. I have seen it and with my daughter. But she is 10. She doesn't even wear camisoles or "training bras" yet, under her clothes. There is no need to, unless she were developing or was wearing a sheer colored shirt.