Of course then they are out of the workforce and 'unhirable' when they try to re-enter it.
Pro tip: Have the full time parent take classes part time and do volunteer work. These days, blogging and online freelance work can also play a part.
Those things help preserve your employability. They give you something to put on a resume. The one and only thing that has the ability to increase your earning power while you take time off work is education. I read that somewhere and I have experienced it firsthand.
I got my first full time paid job at age 41. It paid better than minimum wage and I worked for the largest company in town. Any time I made small talk while buying myself lunch or getting a haircut, telling people where I worked got oohs and aahs. They didn't even know my job title or that I was stuck in an entry level job and never managed to get promoted. Just having a job there at all was statusy and a ticket to a real career.
I wasn't making the kind of money my ex made, but I wasn't doomed either. I ultimately left (for health reasons). I now do freelance work and, with my health issues resolving, my income is going up. At some point, I expect to live comfortably.