There isn't anything they could have practically done to prevent this except not allowing kids to use it.

They may have chosen not to age restrict it because

1) It's really not practical to do that effectively

2) more importantly (and they seem to care about this more than most companies) it would push kids to less safe models like those used on character.ai

What OpenAI does now is what trying to make AI safe looks like. Most of the people arguing for "accountability" are de facto arguing for a wild west situation.