Recombination of one of the adenovirus vector vaccines with a wild strain of adenovirus, creating a hybrid that could infect humans, replicate and manufacture spike protein. The children would have caught this from others -- it will be out in the population now and spreading. Adults will be somewhat protected from it because of immunity coming from prior infection with similar adenovirus strains. Young children have no such immunity and are getting ill.

The adenovirus vector vaccines were J&J, AstraZeneca and Sputnik.

This could be identified by sequencing the virus responsible, but also simply by testing the children to see if they have antibodies to the spike protein but not the N-protein of covid -- this could only occur if they'd been 'vaccinated' by a hybrid adenovirus.