I almost didn't understand I was different gender wise up until puberty at age 10. I knew I was somehow different though at 5. Of course I was bullied and received remarks because I was very effeminate. It wasn't until I was about 17 that I discovered that transitioning was actually a thing, because all I knew off were the terms '->-bleeped-<-' and 'crossdresser'.
I didn't correct people who called me boy, maybe it didn't happen often and maybe it didn't have any meaning to me then. I was me regardless of what people referred to me as. Gradually as I became older it felt wrong and very awkward when people referred to me as male. I lived as an androgynous person for most of my teenage years and decided it wasn't enough after much speculation whether that was how I truly felt, or if society just wanted me to fit a certain box.
Anyhow, my point is that mtf transgender children may not actually claim to be female if that term has no meaning to them. They will definitely show their femininity though, and that they basically behave as girls. It might be harder to 'see the signs' of a tom boy since they would presumably mostly feel good playing with groups of boys e.g., but this is just my personal experience 🙂