You articulated a bunch of things that I was thinking.

At its core, the assumption that a unique and horrible neglect on the husband's part is the explanation for this seems not only unfair, but weirdly naive about the capacity of the toddler, given that it's her second. Perhaps her terror is behind the feeling of blame, and she may come to a more realistic picture of how quickly and easily a toddler can slip away after she's had time, or therapy, or whatever.

And it definitely doesn't sound like any of the many backyard parties with small children in my childhood or adult memories.