No “gender” stuff until they are 18 years old, no exceptions.
Yeah this demonstrates a complete lack of understanding and probably an unwillingness to try to understand. As a cis-gender individuals you and I can’t know what it’s like to be trans, but we can listen to those who do know and try to understand to the best of our abilities. You clearly don’t wish to, but I have listened and I try to understand because I wish to be a decent human being.
My friends and family who are trans have said they knew from very young ages that they are not the gender they may have been biologically born as.
Also once you know you are trans one of the first steps is to take medications that prevent puberty from occuring which allows a much easier and cleaner transition later in life as well as allowing them to present as the gender they wish to present as. Preventing access to this kind of medical care until they are 18 is severely detrimental.
Gender affirming medical care is critical because the suicide rates of trans people who are not able to transition are incredibly high and rates of regret for those who are able to receive transition surgery is so shocking close to zero.
TL;DR you’re wrong and an asshole for choosing not to learn otherwise