Reopening has begun. No one is sure what happens next.

And it isn’t clear what, exactly, it means to gradually restart a system with as many interlocking pieces as the U.S. economy. How can one factory reopen when its suppliers remain shuttered? How can parents return to work when schools are still closed? How can older people return when there is still no effective treatment or vaccine? What is the government’s role in helping private businesses that may initially need to operate at a fraction of their normal capacity?

South Carolina, for example, looks likely to be among the first states to allow widespread reopening of businesses. But if a manufacturer there depends on a part made in Ohio, where the virus is still spreading, it may not be able to resume production, regardless of the rules…

Mr. Vavra and two colleagues recently estimated that nearly one-third of U.S. households have a child under 14, and that more than one in 10 has no other adult in the household to help with child care. In addition, many reopening plans call for younger adults to return to work first, while people over 55, who are at greater risk of severe complications or death, stay home longer to avoid exposure. But younger adults are also more likely to have young children at home.