Given that biometics, transport discount cards and the NCS in school have already profiled them ...
then kids using social media is actually in competition, so yeah I can see why they don't want them on there making it so the child catchers from chitty chitty bang bang can't make a buck.
That being said, since pretty much all Social networks require user confirmation for them to profile and since these kids are underage then they are profiling them without legal consent, never mind GDPR.
Not that I believe they care any more than the children's parents or UK Government do.