The age limit placed on people below 18 year to keep them away frim gambling have not been most effective as we have hard records of under age gambling in many place and this is a pointer that casino has indeed misolaced their primary responsibility which is to prevent that from happening and that is why there is need for KYC.
Casinos have provided regulations by not allowing underage children to gamble. And even though casinos have implemented KYC for their members, that can't stop minors from registering and gambling. They can do KYC using the identity of adults around them, which helps them gamble. Perhaps, this can be reduced if each parent pays more attention to supervising their children using devices connected to the internet so that their children do not visit casinos.
If a minor wants to gamble online you can be sure they can find a way to do it, after all even if their parents are the kind to give a lot of attention to them they also have their own issues, so if they leave their ID and a credit card lying around then their kids could use them to pass KYC on many different casinos, and if I am not wrong this has happened many times already.
So parents have no option but to give the best education they can to their kids and then trust they will do the right thing, as keeping an eye on them all the time is something which is not possible to do.