They have done so for thousands of years.
Where is the evidence for this? I checked life expectancy for the Stone Age. One study of 80 skeletons from 10,000 years ago put life expectancy at 25 - 30. Another put it to 20 - 25. I am struggling with the maths to work out how a 20 - 25 year old could be a grandparent?
By the Middle Ages, infant mortality up to age 4 was high, but if you made it past 4; then life expectancy was 55. Without contraception, presumably women were having babies up to menopause?
Even within my mother’s working class family, one great grandmother had 8 children. Actually all her 8 children commented that having the twins broke her. If you have 8 children, who all have 2 children, how much time could one woman give to 16 grandchildren? Later on, the 8 adult DC had their elderly mother to stay with them in turn; but then care of an elderly person is much easier shared among 16 adults; than today’s average of 2 adult DC.
Likewise, another great grandmother had 7 children. One died in her early 20s from a congenital heart problem. One of the others died, in a back street abortion of baby no 10. The 9 children were shared out among her 5 surviving adult siblings. So, how much time, did my great grandmother have to care for 19 grandchildren?
None of the women in my grandmother’s generation could have relied on full time childcare from their mother, to enable them to work, because her time was spread too thin!
On my father’s side, they were all middle class and married women did not work in Victorian/Edwardian/early 20th century times. Certainly, none of them provided child care to anyone else’s DC (although none of them were fit to - my great grandmother was an alcoholic and my grandmother spent her time, playing bridge and totally neglected my father).