WASHINGTON — Federal health officials said Sunday that kid-sized doses of Pfizer’s COVID-19 vaccines appear to be safe and effective for kids under 5, a key step toward a long-awaited decision to begin vaccinating the youngest American children. The Food and Drug Administration posted its analysis of the Pfizer shot ahead of a Wednesday meeting where outside experts will vote on whether the shots are ready for the nation’s 18 million babies, toddlers and preschoolers.