My friend's mum has been looking after her two dc's for 2-3 days a week for 7 years now (unpaid). She's now trying for another baby which would add a further 4 years to her mother's unpaid childcare duties. They keep falling out over parenting styles and their relationship is nothing like as good as it was pre-children.
It seems to me that it must be almost impossible to keep all parties happy with such a long term arrangement.