For the under 5 age group, Moderna's vaccine is a 2 shot series using 25% of their adult dosage. However, Pfizer's vaccine is a 3-shot series using 10% of their adult dosage. This is for the primary series, not including boosters. In this case, I think the original poster's doctor has recommended or already starting giving the Pfizer vaccine to their youngest. I believe that means they need to finish all 3 shots to be considered fully vaccinated.
