"So at what point does the single stop?"
When they co-parent with you.
It's rare to be a fully blended family, when the children are older and especially after you split.
Women struggle to get the Fathers of their children, whilst living together, to fully share finances and the workload. Very few get the Mental Load shared.
If the Partner doesn't have PR, then most of the educational and health stuff falls on the Parents that do.
Which is why you can't start to pick apart the lives of others.
My DD is a LP, but she has me and my eldest DD on hand. My DD goes out at least twice a week and doesn't have to do every school run/class etc. Her Partner gives her money occasionally, but does nothing else.
Whilst she was with him, she was on her own.
I was married with a DH who worked away, I did all the practical care by myself, as I had no family help.