I don't think we should be either. But asides from the well known differences in brain size and white / grey matter ratios which I'm sure you're already with, I encourage you to visit your local Lesbian Mother's Group where I am sure you will find many parents who absolutely believe in year-0 of sex differences and who, based on my sister's experiences, are often very surprised about how boys and girls act.
> But I suspect that what you're actually saying is that we should not tolerate transgender expression in children.
There is no reason to say this. Many people against gender theory are former 'tomboys' that are healthy adults, don't conform to gender stereotypes, and are glad that breast binding, hormones and cosmetic surgery weren't foisted on them as children.