You basically have to have at least one parent with a flexible job (or a grandparent who lives nearby and is willing to help out at a moment's notice).
Ideally you have one stay-at-home or part-time parent. Problem is, if you both make average incomes, you can't do that. You need both parents working full-time to not be poor.
My kids are still very young (baby and a toddler) and they go to part-time daycare right now. I have no idea how my wife and I are going to make it work when they're older and home for weeks for Christmas break, spring break, and summer break. Let alone all the random days school is closed or they're sick. My mom either didn't work or worked part-time when I was a kid so it was never an issue, but dropping down to one income would break me and my wife.