Not my kid, but my best friends, and I babysit a lot.

Couple things you might not think about, or might overthink.

The biggest one for me was that I kept mis-gendering the kid for like a month, but I didnt realize that how I was reacting to doing so was worse than the misgendering itself.

Generally speaking, people understand that there is a transition period, you have thought of this person as a he or a her for their whole life, suddenly swapping pronouns isnt easy. If you get it wrong, thats ok, dont make a big production of it, just correct yourself, say sorry and try to do better.

I was making sure to really get across that I was sorry, and eventually I got told that it was stressing the kid out, because it was now a big thing that I got it wrong.

I learned, and they relaxed a lot when the next time I misgendered them, I just went "hes a good kid...sorry, shes a good kid" and that was that.