The issue here isn't that children can talk to it. In fact, it's probably a useful tool for teenagers to ask questions they could get in trouble for. Like sex education in more close-minded communities. The issue is that in the example the AI wasn't able to pick up on subtle context clues over multiple messages that a human could. If an adult were told those things they would know something is wrong and could help the child, while the AI can't, even if it would understand