There were, still are, two parents, I don't know why you are focusing on the mother.
I think all those thousands of other parents were equally negligent if they left two young children with an unqualified teenager for long hours. The fact that their children are OK isn't relevant. We don't know of LW was responsible for what happened so we can't blame Matthew's death on the choice the parents made. We can say children deserve better.
Au pairs who do the school run and a couple of hours after school with children are a different matter. Qualified nannies working all day with young children are also a different matter.
@ancientgran
First, why does this phrase 'qualified nanny' keep on cropping up?
There is no such thing in the American context. American parents are not looking for 'qualifications' in a nanny. They are looking for solid experience, and the fact that so many hundreds of thousands of children emerge unscathed from having a nanny is absolutely relevant. The preference for experience among American parents is due to the parents' observation that experience shows both sustained interest in childcare and a realistic understanding of what it entails.
Most professional American parents of the 1990s would have done lots of babysitting themselves as teens, and they would have observed a difference between themselves and those of their peers who did not when it came to ability to work with children and interest in working with children.
I am focusing on the mother because at the time all of this happened, the relentless focus was on Deborah Eappen and the message she received loud and clear was what a terrible mother she was, what a greedy yuppie. It appears from this thread that nothing has changed when it comes to opinions.
And fyi, au pairs in the US can work up to 45 hours per week. They do not just do the school run and afternoon babysitting.