Moderna is asking United State regulators to open its COVID-19 vaccine to the nation’s youngest children. Kids under 5 are the only group in the US not yet eligible for vaccination, and frustrated parents are waiting impatiently for a chance to protect them. Moderna hopes the Food and Drug Administration will rule in time for tots to start getting vaccinated by summer. It’s a complex decision partly because while other countries give Moderna shots to older children, the U.S. so far has restricted them to adults. Rival Pfizer also is studying its vaccine in the littlest kids.