Most probably not. There -are- differences in children, but they're relatively small, and they vanish after just a little while on HRT. Plus, I think that at least 50 % of that difference is due to the undeniable masculine impression we get from short hair and such, and at least 25 % of it comes from the fact that boys typically do different things from girls, e.g. they tend to be more physical, and as such, would have more muscle in average (children's hormone levels don't differ a whole lot from one gender to the other, so I'm sure not all of the muscle mass difference in children can be attributed to hormones).
Anyone needing FFS after very early HRT would have needed it as a born female anyway - they're just very masculine-looking genetically. And such a case is super rare. The very, very vast majority of females don't need any surgery to appear female, and I think it's (almost?) the same for MTFs who haven't lived a male puberty.
That, or she started at, say, 12, but had an exceptionally early puberty, and was already quite masculinised by the time she started HRT. I guess such a thing could happen in a few cases.