I think it's a conversation to have before even trying to have children. It's a bit much to do, like an acquaintance of mine. She and her DH had one child, her parents stepped up and looked after the DC when she returned to work.
His parents did 2 days a week, her parents did 3 days. They saved a mint on childcare. Her mum still worked part time so part of her time off was spent looking after the GC. Her dad was retired. Her DH's parents were both retired. Having saved a fortune they didn't even give the GP spending money to help to take the child out - that was GP choice. And, of course, they spent out on trips. So for four/five years they carried on. She got annoyed when one of the set decided to go on holiday 'outside' a time that suited her and her DH - couldn't get time off work so had to pay someone as the other set couldn't step up.
Her mum then retired and she and her husband decided they'd like to spend a bit more time on weeks away, days out during the week now they had their time back. The GC was now at school and went to a friend's house three times a week after school so, apart from holidays, their time was their own.
Lo and behold DD announced she was expecting and got really nasty when the GP announced they'd help for a couple of weeks in the summer and other holidays and once a fortnight for a couple of days a week. The expectation was 'you helped doing x hours with the first so you help with the subsequent'. No conversation before TTC. Forgetting that the GP were now 5 years older, both retired so less commitments (apart from the babysitting which was over to a large extent with DC at school) and had done their stint by raising her, her siblings and their first GC.
Don't assume GP are going to be there for you. Lives change. My mum developed Alzheimer's when DS was 6 years old. Health can change dramatically for anyone and, sometimes, retired people go back to work part time to help with bills etc. GP should be back up, plan B. Not the basis on which you decide to start a family. Because they may not always be there for many reasons.