Help explaining to a 12yo

[puuteknikko]

Also he asked if a vaccine is just dead flu cells aren't there enough dead SARS-CoV-2 cells out now that we could inject them in to some people?

Vaccine is either inactivated viruses, bacteria etc. or simply weakened versions of them which won't be able to cause the disease in question. Something that the human body will detect as dangerous even if it won't be in this case and starts working on the antibody itself.