He's responsible for a gross oversight in safety that led to an injured child. There were obvious and simple safety precautions that were missed.

This wasn't just, "whoops", this was fairly gross negligence.

Then you don't want a voluntary resignation, I think you want them ousted.

The kind of person to learn from their mistakes through honest self reflection (and hence would voluntarily resign) is probably the kind of person you might want to keep around, or at least not decide to make a public example of.