The FDA issued an EUA for the first Pfizer COVID-19 shots for this age group of children in October of 2021. (Source.)

At the time, a doctor on the FDA Advisory Committee deliberating on whether or not an EUA should be given for this age group, stated that the only way to find out if the Pfizer COVID-19 vaccines were safe for this age group was to start injecting them with it.

"We're never gonna learn about how safe the vaccine is until we start giving it, and that's just the way it goes."