There are two main reasons you want to use specialist clinics for children. The first is primarily because children are more likely to have uncertainty about their identities. The second is because children are developing organisms and have very different bloodtest results depending on what age they are and what part of their development they're in. You need psychological clinicians specially trained in the uncertainty these children have and how to address it without leading them (and deal with parents too) and you need specialists in child development to handle all the weirdness you can get with children at different ages if you're administering something like puberty blockers to hold off their development until they're at an age to decide on transition.

This is fairly good recent research if you're interested in recommended clinical guidelines: https://adc.bmj.com/content/early/2024/04/09/archdischild-2023-326500

Another less talked about reason in my opinion is that it puts all these people that have the same problem in close proximity which means they inevitably meet other parents and other children dealing with it. This has positive outcomes in terms of constructing support networks.