BOSTON — Child development experts and advocates are urging Facebook to pull the plug on its new messaging app aimed at kids.A group letter sent Tuesday to CEO Mark Zuckerberg argues that younger children — the app is intended for those under 13 — aren't ready to have social media accounts, navigate the complexities of online relationships or protect their own privacy.Facebook launched the free Messenger Kids app in December, pitching it as a way for children to chat with family members and friends approved by parents. It doesn't give kids separate Facebook or Messenger accounts. Rather, the app works as an extension of a parent's account, and parents get controls such as the ability to decide who their kids can chat with.The social media giant has said it fills "a need for a messaging app that lets kids connect with people they love but also has the level of control parents want."