I can see arguments that it's worse for the family, but why is it worse for the economy to have two parents working?

Suppose childcare is $15k/year and you work minimum wage making less than $15k/year. Then there's less wealth to go around, just more in your pocket. But actually, you probably don't take home all the wealth you create, so it can actually still be better for the economy. It is still worse for the economy, but not for that reason. Probably because labor has a backward-bending supply curve, and most people are already working more hours than is optimal. As another commenter said, it would probably be better for the economy to make a 30 hour work week.