There are several factors to consider
Pfizer released data suggesting that their vaccines are well-tolerated in adolescents and highly effective in preventing Covid-19. Canada, the US, and the EU have already authorized the Pfizer vaccine in children as young as 12. And the UK has just approved the use of the Pfizer vaccine in children aged 12 to 15.
Whether a vaccine is beneficial for someone depends on three things: How likely they are to become seriously ill from the infection, how effective the vaccine is, and the risks of vaccination.
Children have less severe Covid-19 than adults. A recent study across seven countries suggests that there were less than two in a million Covid deaths in children. Even if the vaccine is highly effective in children, it can only eliminate an already tiny risk. Of course, there are other problems, such as long Covid, but we don’t know yet how common this is in children.