This again.

NOBODY does irreversible physical intervention on kids under the age of 16.  If a kid thinks he or she is transgender, at most they'll be put on puberty-delaying drugs.  When they're mature enough to decide, that's when irreversible intervention can begin.

And yes, kids should be believed.  Cross-gender behaviour in kids is completely normal and not an indication of gender dysphoria, but any child showing severe distress over his or her assigned gender had better be believed or he or she will suffer miserably.