Provided their parents are aware and grant permission, there's almost no limits to what they can do.
Obviously, they can't do 18+ stuff, and most sites state in their T&C that users under 18 must have the permission of their parent or guardian.
I've employed teen writers in the past, and one of my nephews started vlogging on YouTube (monetised) when he was still a minor. His dad was cool with it, and it was monitored, so no worries.
Personally, I'd suggest GPTs and, if they're serious, monetised YouTube. Teens with confidence are popular on YT, since they're in with the in-crowd and YT has a stupendously large, young demographic.
The gig sites would work, especially if they have a particular talent to market.
Writing is also great, but it's not as easy to find work or the perseverance to do your own blog at that age, so it might not be such a good option.