That isn't actually the norm.
It's only one example of the various "norms" that exist today. Families with 2 working parents, families with 1 working parent, families with 1 parent period - all that and more exists in thousands of places today.
I was actually curious, and ran across this article:
https://www.washingtonpost.com/news...nce-the-1950s-there-is-no-typical-u-s-family/
Apparently, today, children are being raised in the following situations.:
22% of families are the "traditional" one working parent household.
34% are two parents working.
23% are a single mother raising a child or children.
7% are a single parent living with an unmarried partner - it's not clear in the article how many of these partners fill a parent-like role.
3% are a single father.
3% are being raised by their grandparents
Based on this it would seem that at least 1/4 of all households with children to support are single-parent. It is possible they could survive on less than $15/hr, if full time, and depending on location, but I don't know if they could survive on 7.50/hr.
And if they can't pay all the bills on $7.50/hr, they'll likely need assistance. Probably from the government, in the end.