One challenge with hiring a nanny is if they need to take a sick day (or if they quit!) you can end up in a tough spot. In contrast a day care center usually has backups built in so you don’t end up scrambling.

Because a nanny in Seattle charges $30/hr.

Also, a nanny is not the same ecosystem as child-care. It's not nescessarily a substitute. If what's desired is a private-private child-care, I suspect you'll be paying a lot more than the price of a nanny.

A private nanny isn't realistic for 4 kids. You need 2.