To use a crass analogy, this is like saying that we need to be sure that immunisations don’t cause autism before we inject them into children.
No, it isn't.
What I'm highlighting is a genuine epistemic issue, whether or not children "know" that they are transgender. This has been asserted on the basis that the majority of transgender adults report that they "knew" they were trans in childhood. But this not a straightforwardly accurate claim, because what we are actually describing is that adult trans people have retroactively interpreted
their childhood or adolescent experience as knowledge of being trans. Very trans people can honestly claim an explicit awareness of being trans as children, of having through "I am a trans person"; what they are describing is a sense of longing or of identification with another sex or gender, experiences which they have later identified with being trans; the "knowledge" is presented as intuitive rather than as intellectual. This isn't invalid on an emotional level, but it's unreliable as a factual account: adults are notoriously quite bad at providing accurate accounts of their childhood, because the way we habitually organise childhood memories is to make sense of our current identity. We construct narratives that lead to where we are now, and we can reasonably expect that people who have struggled to achieve a secure sense of identity would be more likely to organise their memories into such a narrative.
Further, we are working from a biased sample: people who transitioned as adolescent or adults, and so for whom childhood "knowledge" of being trans was later validated. Even if we disregard everything I've written above, we are still failing to account for people who exhibit dysphoria and/or gender non-conforming behaviour in childhood or adolescence, but do not transition as adults. The experiences or behaviours which are cited as demonstrating childhood knowledge of being trans do not universally correspond to later transition as an adult. It stretches credibility that children who later transition would almost universally possesses knowledge that they are trans as children, but that other children with very similar experiences who do not transition would not possess this knowledge; this would either require us to attribute to children and adolescents some uncanny predictive power which is inaccessible to adults, or it would require us to discover some additional facet or layer of a trans identity which is obvious to children and adolescents but has so far escaped the view of adult researches, both cis and trans.
This strongly suggests that the experiences that adult trans people report as "knowledge" of being trans are rendered as such in retrospective knowledge that they would eventually transition; had they not transitioned, the same experiences would be interpreted differently. I am wholly sympathetic to the idea that later transition helps make sense of childhood or adolescent experiences, but I'm sceptical that this is something which is straightforwardly discovered
about those experiences, let alone something which was known at the time
. In some cases, it may be true, but self-reporting years or decades after the fact isn't reliable proof of that.
The takeaway from all this is that children probably do not
know that they are trans, and that we do not know which children who exhibit emotions or behaviours which are consistent with the reported childhood experiences of adults who will transition will turn out to be trans and which will not. If we cannot, then, reliably predict which children or adolescents exhibiting these behaviours will permanently transition as adults, we cannot reliably identify which of them it may appropriate to facilitate or encourage to pursue transition as children or adolescents. Caution is merited, and caution should increase with the risks of medical intervention; "risks", here, not describing some mysterious second ailment, but the straightforward implications of interrupting somebody's hormonal development, and which increase if the use of puberty blockers is assumed to represent the first step in a more comprehensive set of medical interventions, as some posters here, and certainly a lot of commentators elsewhere, seem to strongly assume.
I hope the above shows I've given this enough consideration that comparisons to anti-vax conspiracy theorists is, at least, uncharitable