When Dr. Joshua Ishal got his 5- and 7-year-old daughters their first doses of the Covid-19 vaccine last week in Queens, he joined millions of other parents in protecting their 5- to 11-year-old children since the Pfizer-BioNTech vaccine was authorized for this age group in late October. Dr. Ishal, a dentist who lives in Great Neck, N.Y., never questioned whether he would get his children vaccinated, but he has been wavering over the timing of their second shots. The clinical trials that tested the Pfizer vaccine separated the doses by three weeks, which is why the U.S. Centers for Disease...