Gender identity develops somewhere between three and five years old.
And again, there's no harm letting them experiment with gender identity and presentation.
Only a small minority of kids who transition before puberty chose to detransition and while this number is higher amongst kids who express a transgender identity before the age of 6, one study shows that the total is 7.3% of youth "retransitioning" (that is to say, changing their gender identity). 94% of these kids were set on a trans identity, 2.5% returned to their assigned sex and 3.5% left the gender binary altogether. The majority of the young kids who did retransition did so prior to puberty, meaning they did not receive any medical gender affirming care.
It's very common to mistakenly believe these kids don't know their gender identity because you don't understand the age at which it develops...most people don't including many pediatricians.
They see it as the kid doing the same thing as calling themselves a cat, but the truth is if they're just saying it to say it they won't persist when asked if they are sure, whilst trans kids will keep saying it until either somebody listens or they become repressed, which leads to poor mental health outcomes and higher rates of suicide.
At this age, "transitioning" means using a different name and pronouns, cutting your hair in a different style and wearing different clothes, all completely harmless and reversible. And I'm not saying you should transition a kid who says they're the opposite sex once or twice. Obviously parents should spend time with the kid and work out whether they are sure about it and whether this is a consistent identity thing.
But the harm of not letting them experiment with gender identity is far worse than letting them do it.