We discussed this with my girls when they were teenagers - and it was clear to them right from the start. I shall always be there for them if and when they decide to become mums (one has, one has not). I would help, support, always be there in an emergency, have kids for holidays - but would never bring up their children for them (unless something terrible happened) - and they totally agreed. I so love our grand-children, but their parents are responsible for their care and for suitable child-care when they are working- with us a emergency and occasional support.
You have every right to work full-time if you choose, and I sincerely hope that they will not take it too badly if and when you tell them about this. Thinking of you.
To think that London, or anywhere else for that matter, does not belong to any one demographic