They can deceive grannies to do this when they have a good relationship and afterwards use their accounts for their preferred purposes, the parents can offer parental controls in the emails in their devices and ensure those emails are not replaced by these young ones in order to stay abreast with their activities, else the more restrictive approaches by these companies, the smarter these lads are getting to bypass those security concerns.
As long online casinos and gambling sites are legal and easily accessible to everyone who owns or possesses smart devices and computers, teenagers and underages will always sort for ways and alternatives to gain access to them, regardless of the restrictions placed on those sites or their devices to stop or prevent them from participating in these activities.
Just like you rightly stated, the more you restrict them the smarter they become and the more you try to prevent them from doing something, the more you raise their curiosity and the more they become even more inquisitive to participate, so rather putting all that energy in tryna restrict them, it'll produce a lot more results when that energy is channeled on educating them on the possible dangers of gambling and how gambling can destroy a person's life who do not possess the required experience and skill to be a gambler.