Parents and would-be parents might be relieved to hear that last week's widely reported government figure estimating the cost of raising a child—around $300,000 over the first 17 years for middle-income, two-parent families—is, according to several economists, wide of the mark.

The bad news is it may be a severe underestimate. The U.S. Department of Agriculture's report leaves two things out of account. For a start, it doesn't reflect unpaid time spent on parenting, including income forgone by parents who cut back on work...