Is it going to harm anybody's cis kid? Is it fuck.
Yes, yes it is. It will harm 'cis' kids when they hear 'I knew I was a girl because I always liked pink/princesses/shoes/glitter.' Or 'I'm a boy because I like football and not dolls.' Or 'Girls can have penises and boys can have vulvas.' Or 'the gender that you feel is more important than your biology.' Or 'Your feelings as a 'cis' person are less important than trans people's wish to belong.'
All of these messages are harmful.
The message that we should be sending is:
'We can't change your biology. If you were born a boy, you can't ever be pregnant. If you were born a girl, you can't ever become biologically male. But no matter what biological sex you are, you can choose what to wear, what to play with, what job to do and no one should EVER discriminate against you, bully you or prevent you from making your own choices. And if this ever happens, others will stand up with you and fight for your rights.'