Dear Libby:
The reason your school, and your teachers' union, and the majority of the staff, have safeguarding guidelines that say "Don't out a LGBT student to their parents unless you have the student's explicit permission to discuss this with the student's parents" is the same reason doctors aren't allowed to discuss a child having an abortion with the child's parents.
If the child has made the decision that it is not safe to discuss these issues with the parents, the child has made an informed decision based on much more information about their parents' characteristics than the teachers (or a doctor) can have.
Allowing a child the freedom to say what pronouns the child uses, and what name the child wants to be called by, is a harmless freedom: there is no reason at all for the parents to be told the child has picked the name Dennis when the parents always call their child Claudia, any more than there would be if the child says "Call me Denny" and the parents have always called the child Dennis. The parents might want to know that their child no longer uses "Dennis", but they don't need to know.
Especially as there is always a risk - a real risk, even if a small one - that the parents will turn out to be bigoted against trans people, will react badly to discovering that their child is considering if they may be transgender or nonbinary. There is no reason to put any child at risk when it's completely unnecessary. As your trade union, and your school's safeguarding policies, and most of the other staff, have already let you know.