Why a vaccine for younger children takes longer

Hundreds of millions of adults have been vaccinated, proving that the Covid-19 vaccines are safe and effective, but those results are not a substitute for the research needed in kids.

"As much as we would like to go ahead and start vaccinating our children now, it's most important that we take this time to ensure that the science is rigorous," said Minnesota's Chapman.
For the kid's version of the Covid-19 vaccine, scientists use results from the adult trials and a full pediatric trial.
Having the adult research speeds up the process. For people as young as 12, Perlman explains, the companies didn't have to enroll the 30,000 people it needed for adult trials because it could do what's called "immunobridging." The data showed that for this age group, the immune response was the equivalent of adults'.
age group, the immune response was the equivalent of adults'.
Companies take a similar approach with the younger kids, but in early August, the FDA asked for six months of follow-up safety data, instead of the two months it asked for with adults. It also asked Pfizer and Moderna to double the number of children ages 5 to 11 in clinical trials.
Vaccine advisers to the CDC said in June there is a likely association between the mRNA Covid-19 vaccines and extremely rare cases of heart inflammation in adolescents and young adults, but the benefits of vaccination still clearly outweigh the risks. The inflammation cases appeared to be mild, and they resolved quickly on their own or with minimal treatment.
At Texas Children's Hospital, interim pediatrician-in-chief Dr. James Versalovic said it was no problem to recruit more kids for the Pfizer and Moderna trials. Many trial sites have long waiting lists. The trial expansion, though, added at least a month more to the research process.
"We all agreed it was worthwhile, just to make the trials even more robust data to provide that additional level of reassurance to parents across the country. It does lengthen the trial, but just a bit," Versalovic said
 
https://www.cnn.com/2021/09/02/health/kids-covid-vaccine-timeline-wellness/index.html