I have a different view as I changed my first and surname from my deadbeat dads name.
My father filled out my birth certificate and the narcissist made me a junior (think Alex Smith & Alexa Smith) which had never been the plan. I have gone by the name I was suppose to be called all along (so by Kat Jones instead of Alexa Smith), no one in real life has ever called me by his name. However it was always defaulted to for forms, legal stuff, school or medical stuff.
I wanted to change it all my life but wasn't allowed too until 18.
I finally did after I turned 18 but it was delayed didn't officially come through before I had my oldest child and I wasn't allowed to delay registering him.
His birth certificate literally has a name I fucking despise on it, that isn't my name and isn't on anything else (so does not match my passport, drivers license, bank cards etc...), that I have never even gone by... result being it looks like I'm not his mother because my details don't match his birth certificate.
Only way to change it is to have a sex change.
By 18+ you are much more likely to have lifelong documents under that name and its much harder or impossible to change them. It is why lots of women with published career (writers, researchers etc...) or academic certificates (Dr. etc...) choose not to change their names after marriage because it can cause issues with things lining up.
A toddler wanting to be called 'sparkle-fairy' you can laugh off but a child that has gone by a name consistently for years and is old enough to understand the process they likely aren't 'playing' around. Identity is a big thing and your name should be a reflection of you not whoever named you.