The thymus comes to mind as a factor. As we age, the thymus shrinks. Children have a larger thymus which aids in combating viruses. Perhaps if these children had been exposed earlier, no hepatitis would have occurred.

I'd like to see if these cases would have occurred anyway in the preceding years. It may be that cases that were normally(hypothetical numbers for example purposes only) 1/1000 before lockdown went to 1/10,000 during, and after, went to 1/100. Taking the average over a 10 year period, this may be totally expected and not too concerning. If this is true my first point is incorrect.