No one should be monitoring children for more than six hours at a time. A childcare person on their seventh hour is too fatigued to be functional in an emergency.
That said, more than a 20-hour work week will start taxing adults so they can no longer serve their functions regarding civic duties (voting, staying knowledgeable about issues so as to govern policy, etc) and parenting.
Yes, because the most important thing in life is to have nice shit and money.
Also, the fact is that people are more productive when they have better work/life balance, as we can see with the high productivity numbers in Europe vs the abysmal productivity in the US and China. If people are paid for their productivity, as you're implying, people working 6 hour days should actually be making the same as someone working 8 hour days, assuming an efficient free market.