If anything, you confirm the parent's argument. Instead of your wife breastfeeding your children and "wasting" time t for that, she first pumps her milk, after

If anything, you confirm the parent's argument. Instead of your wife breastfeeding your children and "wasting" time t for that, she first pumps her milk, after which you feed them, hence taking about time 2t. Hence, everything else being equal, you're worse off (in economic terms, because time is money) with her not breastfeeding.