With many children now being raised by other family members at least in part, I think your question is very interesting. Of course, in the past, many children were raised in multi-generational households. However, I suspect your question is more about the situation when both of the child's parents are unable or unwilling to take on the responsibility of raising the child, and the grandparents step in to do the job. Here are some other ways to look at this topic:
1. What causes grandparents to become a child's main caregivers and what effect does that have on the child?
2. What effect does it have on grandparents when they need to become primary caregivers of their grandchildren?
3. What is the effect of growing up in a multi-generational household?