That’s phase 1 that is in a very limited number of subjects. The large dose was the full adult dose of 30 mcg. Phase 3 is the large study. They dose picked for the Phase 3 trial for children 5-11 was 10 mcg which is one-third the adult dose. I see that they also had an experimental trial for a lower dose for the 16 - 30 age group.
A Phase 1/2/3 Study to Evaluate the Safety, Tolerability, and Immunogenicity of an RNA Vaccine Candidate Against COVID-19 in Healthy Children and Young Adults
That is the name of it, Phase 1/2/3. They don’t state on that page how many participants.