That’s questionable

According to your own sources, and others, Multisystem Inflammatory Response has been more commonly observed among patients who did not receive vaccinations, and became ill with the virus after exposure to people already infected.

Cases were first noted in children as early as July 2020, before any vaccines were approved or distributed :

https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(20)30271-6/fulltext