I think OP missed the fact that clinical trials for COVID-19 vaccines in children measured neutralizing antibodies as the surrogate endpoint, similar to clinical trials for other vaccines [1]. So it is not surprising that the trials could achieve their primary endpoint while being inconclusive on preventing infection. On top of that, the benefit of vaccination can arise not just from preventing infection but also from reducing disease severity.
[1]: https://www.fda.gov/drugs/development-resources/table-surrog... "Table of Surrogate Endpoints That Were the Basis of Drug Approval or Licensure"