The issue is with testing vaccines on children because they can't give consent. There are very strict guidelines on this.
Ideally, children should be vaccinated because they can still transmit infection. It's claimed that vaccines reduce, or maybe even eradicate, infection so it's necessary for this link in the infection chain to be broken.
Looking at the cases over the last week by age group, the 10-19 group has the highest number of cases of all age groups, undoubtedly caused by the full-time reopening of schools. Although the children themselves will probably not be badly affected, they will take (and already have taken) infection back to their families, some of whom will be more vulnerable.