They did placebo for the safety side of the study, not the efficacy side. If they were doing a efficacy study with placebo would have required a much larger sample size and time. Note that it required 30,000 adults, 15,000 in each group, to establish efficacy for adults. It required that many to make sure they had a strong signal to noise ratio.
There are not enough kids enrolled in the study to have enough data points to get meaningful results in any reasonable amount of time. In fact, because kids are even more likely to be asymptomatic, it might even require more kids than adults to show efficacy using a placebo control group.
So while they gave some kids a placebo, that data was not used to prove “100% effective” in adolescents.
