This is a non-issue for most asian countries. Grandparents and women who couldn't keep up with men in professional settings become caretakers. China's biggest issue is its population bomb not a lack of people to watch the kids.
It isn't just at a young age that kids need watching, and time isn't the only resource invested in children.
Think of the percentage of any modernizing economy that goes into things like food and clothing for children, education and educational materials, transport, energy consumption. The reduced output of parents who are tired from looking after or arguing with children. And this is for something that won't start putting value back into the economy for a good 18 years from initial investment.
Skip this, and instead of food and clothes for children adults can spend on housing, transport, take risks with starting businesses without the chance of hungry dependants, etc
By productive if you mean only time-wise, don't forget that single kid families can increase consumption of goods with the saved Yuan leading to higher GDP as well.