I benefit just as much from parent's children (heh) as I would from yours, but less than I would from my own, because they'd entertain me.

Parent's enjoyment of those children is an appropriate benefit which both parent and parent's wife have purchased at the cost of parent's wife's career and their retirement savings.

Put another way, parent and I both receive an economic benefit, but parent receives an emotional benefit. You can argue that I should pay for the economic benefit I receive, but I do not think it appropriate that I should subsidize the price of his emotional benefit.