Just to say she doesn't " need" support at all unless she wants it IYSWIM. Going braless doesn't mean droopy boobs when you are old ( gettin old means droopy boobs in it'self IYSWIM!).
Just buy bigger crop tops if that's what she wants. If you scout the shops first or bring some home to try that you can take back, you can go for the more " sports bra" style ones without her really noticing too.
DD age 10 has probably an A cup and is more developed than most of her friends. We have just moved house and she is starting a new school in a few weeks. In old school many girls wore crop tops, but her's are now too small. I have asked if she wants some new ones or anything different and she burst into tears, worrying that she will be different to the girls in her new school. Suspect she needs a bra now but think the suggestion would horrify her. Any suggestions? Suspect once start new school she will see that the other girls are similar to last i.e. some more developed than others. however she does not like to wear a jumper and a shirt with nothing under it is no longer suitable.