What is a full time parent? I remember as a kid that I was always encouraging my mom to take up hobbies so that she's leave me alone so I could get on with my own stuff. I can't imagine putting all your identity into being a parent. Kids grow up extremely fast.
All the more reason to be a full-time parent. You get a return on your investment extremely fast. When you're old, you have adult kids that care about you, and hopefully grandchildren which are basically just children but free because you're not the primary caretaker anymore. All the fun with much less fuss.
I presume GP doesn't mean someone with no other identity, just someone who is doing it instead of a FT paid role. I could be a full-time plumber and still have an identity outside of plumbing