For children who are transitioning, they are not allowed to go on hormones until they are an adult. They would be on puberty blocking drugs, or would have gone through puberty as the sex they were assigned at birth. In the case of the former, they would not receive any benefits from their body operating with an increased amount of testosterone. For the latter, well, I highly doubt someone would be able to pass themselves off as the other sex having gone through puberty.

For adults, in order to have enough of an appearance of the other sex, you would have to be on hormones for quite some time. There's a wealth of literature out there to show that it does not take very long being on hormones for these differences to completely disappear, even when the child went through puberty of their sex assigned at birth. There are testing protocols which exist where you can confirm that they have estrogen levels high enough to confirm they have been transitioning for an adequate period of time. All of this is entirely irrelevant, however, when it comes to children.

To be clear, my statement was directed at people who do not understand how transitioning works and are not experts in the field. This has been addressed adequately and understood at a professional level for quite some time. The problem is the layperson who has no understanding and does not wish to educate themselves attempting to debate something of which they are ignorant.