Meta Platforms has spent months trying to fix child-safety problems on Instagram and Facebook, but it is struggling to prevent its own systems from enabling and even promoting a vast network of pedophile accounts.

The social-media giant set up a child-safety task force in June after The Wall Street Journal and researchers at Stanford University and the University of Massachusetts Amherst revealed that Instagram’s algorithms connected a web of accounts devoted to the creation, purchasing and trading of underage-sex content.