We know, and the FDA has acknowledged, that Covid presents little risk in children. For Omicron, we have found the vaccine is basically useless (it has been widely reported that the vaccine was just 12% effective in children aged 5-11). And to the extent we are concerned about very rare serious illness, there's no evidence that three shots is better than two. To give you an idea of how rare serious illness is in this age group, we can look at Israel data, which tracks infections by age and vax status. At Omicron's peak, there were 62,000 infections among unvaccinated 5-11 year-olds and just 1 serious infection. For the 10,000 vaccinated, there were none. Indeed, for vaccinated (without a boost) people under age 40, the rate of serious infection has been less than 1 in 100,000 through the Omicron wave.
What exactly then is this booster recommendation based on? Not much. According to FDA's own press release, it's based on increased antibodies (no proof of actual decreased infection or serious infection) in a mere 67 kids. Safety (side effects) was studied in a larger group of 400, but 400 still isn't nearly enough. Myocarditis, which the vaccine appears to cause in about 1 in 3,000 teenage boys, would greatly outweigh the benefit if it occurred in younger children even a fraction as frequently. Younger kids do have a one-third dose, which might reduce the risk of myocarditis, but it might also reduce the already-tiny benefit of the vaccine.
Paul Offit summed it up well in an article in Reuters.
"Dr. Paul Offit, a pediatric infectious diseases expert at Children's Hospital of Philadelphia, said booster shots in the age group would have a limited impact on the pandemic, even with evidence that the additional shots will protect against mild illness for three to six months.
"What is the goal of this vaccine? Are we trying to protect against all symptomatic infection for a limited period of time? Or are we trying to protect against serious illness, in which case all the evidence is that we are preventing serious illness," with the two-dose vaccine regimen, Offit said."
The FDA and CDC began the approval process with a high standard (50% effectiveness) and a huge study (36,000 people for the initial Pfizer randomized study) that showed high effectiveness. For the boosters, they have increasingly relied on scant experimental data and short-term (and sometimes weak) results. This latest approval is unfortunately no exception.