Cosette knows that Instagram is the most widely used social network for teenagers and kids, which also means it's the most attractive social network for online predators to find and befriend kids on. So Cosette created special profiles, and posted images with the kind of tags these predators would search on, so that they'd find these user profiles. The grid layout of Instagram this becomes bars on a prison cell door.
I'm assuming predators don't search on the tag #underage. I mean, do teens really post with that tag when they're taking selfies? Really?