I'd like to think I'd never lose or forget a child. BUT, I say that as the mother of a 1yo constantly-watched-by-me pfb. By the time you have several children and they have a little independence (i.e. an 8yo taking herself to the loo), I bet it becomes easier to imagine how this can happen.
I know someone who moved house, only realising she had left her 11mth old DD in the old house when they got to the new one. It sounds like a horrendous case of neglect- but actually she was the youngest of four DC, the parents had a car each both with a carseat in, the cars were loaded to bursting as well as the van, the DD was playing in an empty room with her 4yo brother who wandered off to watch the van being loaded, leaving the baby to curl up on the floor and fall asleep! Both parents thought the other had her. They are lovely and responsible parents, and they made a mistake. I can't imagine at the moment how you could let it happen, but happen it did, to otherwise very experienced and vigilant parents when they were under stress.
DC and SamC made a parenting mistake. I think we should focus on his political mistakes when assessing his fitness to run the country.