So for just 3.5 kids, you'd need to pay 100% of a salary plus all the extras of running a business. (insurance, legal, finance, advertising, sewage, electricity, food, toys, etc.)

Given the overhead, you'd expect daycare for 2 kids to consume the working parent's entire pay. For a low-paid parent, just 1 kid might consume the entire pay.

Obviously that isn't viable. I know a handyman in Massachusetts who had at least 11 kids. It's perfectly legal for his wife to care for 11 kids.