It's less rare than you might think, here are some statistics from US insurance claims, which is a lower bound as the wealthier may choose to pay privately:
Those numbers are still "rare" and don't tell us that kids are being coerced.
My assumption for now is that the increase is due to latent demand and wider awareness & acceptance. We need to see the steady state after a decade or so.
Several of my high school classmates (from the mid-2000s) started transitioning as soon as they could skip town and decide their own life. And it was clear they had known and wanted it for a long time prior.
https://www.reuters.com/investigates/special-report/usa-tran...
Note this includes surgery on girls as young as 13 years old, to remove their breasts.