However, they won't be starting HRT until puberty from what I understand, so effectively you're just letting your child experiment with their gender identity, which is fine
In some cases it's not even actual replacement therapy that they perform (at least in the case of male to female transitions).
It's possible now for treatments to prevent male puberty, so that the boys don't develop a male body, instead remaining skinny and mostly androgynous as pre-pubescent children are.
My qualm with it though is that boys start puberty anywhere around 10-13, and that's a very young age to make big decisions regarding your body.
It's a difficult thing for me to comment or judge upon however, as I've never felt gender dysphoric at any age, nor had a young child who did. I would imagine that in the future if I had a child who felt dysphoric at a young age (or simply just preferred being a girl to a boy), I would probably be against them having those sorts of treatments until they're 18+. But I couldn't say unless I was in that position.
Anyone interested in this topic can find a rather interesting documentary by
Louis Theroux which came out this year.
It's not available on iPlayer at the moment, but I'm sure you can find a copy of it somewhere online with a bit of googling.