Practically nonexistant, it is not possible for children under 16 to access HRT, if anything they are prescribed puberty blockers (so that you know… they can avoid suffering from the changes in their body and not be permanently affected).

The general figure of regret for transgender affirming care is 1%, and 82.5% of these people do not detransition because they are no longer trans, but because of external factors. Source: https://pubmed.ncbi.nlm.nih.gov/33794108/

By age 17, 0.1% of trans children get HRT. Source: https://jamanetwork.com/journals/jamapediatrics/article-abstract/2828427

Now let’s calculate some numbers: 1% of these 0.1% will regret it, that’s 0.001%. And 17.5% of these might not be transgender, so that’s 0.000175%

According to that second study, there’s about 300000 trans children, so 0.000175% of that is 0.525 people.

So… less than 1 child in the whole United States would maybe regrets it because they’re not trans. In other words, you can stop fear mongering.