It’s a patchwork system of commercial facilities, home-based providers and nannies, plus the grandparents, neighbors and friends who fill in the gaps. Child-care assistance for working parents isn’t guaranteed in the U.S., as it is in many rich nations, and more than half of Americans, especially in low-income and rural areas, live in child-care deserts, where demand far exceeds supply. Cost is another big issue. Infant care, the most expensive type, exceeds 20% of the median household income in 21 states. Caregivers — who are mostly women and disproportionately women of color — earn $12.12 an hour on average for watching children younger than 5, and about half of them rely on public assistance, according to the Center for the Study of Child Care Employment at the University of California, Berkeley. The low earnings lead to higher turnover, further destabilizing the quality of care.