Blaire's underlying assumption which justifies her viewpoint on why transitioning for kids is a deplorable idea: that kids are not in a position to know better, because they are simply not mature or knowledgeable enough to grasp the consequences pegged with the entire transition process. The thing is, age does not fully equivocate or represent one's state of maturity and knowledge. There are mature, intelligent young adults who choose to make such decisions because they are absolutely sure of their gender identity and want to actively take charge of their own developmental process. Just because you think they are too immature to make such decisions at that juncture does not necessarily mean they are. Please keep in mind that outliers do exist too.
Furthermore, Blaire disregards one of the strongest cases for why professionals or parents permit their children to undergo early transition: not because they can pass better in the future (honestly, that seems kind of inconsequential when plastic surgery and hormonal therapy treatments have improved by leaps and bounds over the years) but because they are genuinely afraid of their children being traumatized over the effects of puberty. Besides the gender dysphoria these kids would be facing, the real physical changes, thanks to puberty, that vehemently deny these children their gender identity can cause irreversible trauma to them as well. Other trans folks have mentioned how difficult and painful puberty had been (princessjoules, case in point) for them, and parents (also major stakeholders in this whole debacle) of trans children have voiced their concerns. A mother of an eight-yr-old girl diagnosed with gender dysphoria (I'm lazy to link it here but it's up there on YT anyway, take a look) did mention that her child was taking nail clippers to mutilate her own genitals at an early age, acting out in school etc. Is it really humane for anyone to further compound the child's misery by denying him/her a temporary reprieve from the inevitable changes caused by puberty (by using hormone blockers)? I'm not a great fan of allowing way too early transitions either, but I think active steps to put off puberty till about age 17-18 for kids with gender dysphoria seems like a fair deal- give them the time and space to make their own decisions without further destabilizing or hurting them psychologically seems like a great way to go. Whether or not more research can be delegated towards producing more effective hormone blockers would be a whole other issue on its own anyway.
That being said, Blaire brings up a couple of key points, the irreversibility of transitioning and the permanence of such decisions etc. They are fair points, but Blaire needs to consider the proponents of such arguments a lot more seriously than she did in the video.