it wouldn't be ok for someone to constantly point out to you and your child that you hadn't given birth to them would it?
No, it wouldn't, but I am legally their parent. And in general parlance, we don't call people "birth parents" and "adoptive parents" but, if I'm at a mums group, and everyone gets discussing birth experiences, I don't lie and make one up. So in that respect I am unlike the transwomen who claim to have periods. And there are daft biology-denying adoptive parents who claim to have had sympathetic labour pains but we either ignore them, or point out they are gaslighting their children.
And my children (though they are young, and teenagers often don't want to do this) are quite happy if a peer is saying "I grew in my mummy's tummy" to say "I didn't, I grew in X's tummy".
A child who identifies as trans is much more equivalent to a foster child. Foster children are living with a carer who isn't legally their parent, and they will typically call them by their first name, reserving "mum" for their actual mum.
So if you say to me, "that's not your real child" or "you aren't their real mum" that is daft because they are definitely a child and they are definitely mine and I am definitely a mum and I'm made of flesh and blood.
But if you say "that's not your daughter" to a foster carer, it's true. And they will say, "X lives with us and she's part of the family" which is also true, as "family" is looser.
But if you say a girl who identifies as a boy is a "real boy" then that's not, biologically, true. Nor, in the case of a child is it legally true.