personally I would't go full in, first ask them why they are doing it, it may not be for the same reasons you do or think. But never be pushy, teens and preteens in particular are extremely self-conscious about these things (I was abdl since age 13 and i had to hide and suppress it for over a decade due to a knee-jerk reaction from my mother back then) If they don't wish to tell you then back off for a while and approach again at a later time. Are they Neurodivergent and are you Neurodivergent? (ASD, ADHD, etc.) if so it could be a coping mechanism from overstimulation and anxiety. Point is don't expose them to your thing more than they need to be while helping them discover themselves, there are worse things people get into when stressed at that age (drugs or self harm), so make sure to be supportive but never ever pushy. And honestly if they get exposed to this kind of thing its better that they talk to their parents rather than go on the internet just to learn of some really fucked up variations and individuals that would make them feel ashamed of themselvfirst