There's no evidence of any risk of mRNA vaccines at all. The only issue so far has been extremely rare blood clot issues with the adenovirus vaccines (AstraZeneca & J&J).

The vaccines are not approved for children under 12 because the risks to young children from the virus are the lowest of any population group, so older groups were prioritized for efficacy and safety testing. It is being tested now in that age group (I know people with kids in the trial group) and will hopefully be approved shortly.

If this virus worked the way the flu does, doing the most damage to children and the elderly, then the vaccine would have been tested on children much sooner and would have been available to that group sooner. In that world, that's not because the vaccine was more dangerous to 25 year olds.