Do the things you like determine who or what you are? Does it mean that adults who like toys are children? Does it mean that men who like make-up are women or female football players are, in fact, male?
If you say no to any of those, why is it any different if someone is transgender? If a transgender girl likes masculine activities, she's told "see, I knew you weren't really a girl." However, if a "normal" girl likes the same things she's a hot tomboy.
If a trnsgender guy a likes fememine activities he's asked "how are you a guy if you like those things?" However, if a "normal" guy likes the same things he's in touch with his feminine side.
In either case, a "normal" person liking something doesn't change who they are. people who are transgender are normal people whose bodies simply don't match their brains. We're backward.
Imagine how hard it is to figure out how to function with a backwards body, finally finding your place and having it taken away by people who ask the aforementioned questions.
I'm no expert but I'm pretty sure liking princesses won't turn your little boy into your daughter. So again I ask, do the things you like determine who or what you are?