What is the percentage of children developing multi inflammatory syndrome after being exposed to covid 19? and what is the role of blood type in the severity of symptoms ?

Low incidence : The incidence of this syndrome in children with Covid is quite low. Regarding blood type , they have found a slightly increased risk with blood type A and a slightly reduced risk with type o positive , but this is not absolute . the reason is not clear , yet and may be related to antibodies.

COVID 19: I am sorry but I was not able to find a percentage listed in any of the articles I have looked at. The number is small. Please see: https://coronavirus.health.ny.gov/childhood-inflammatory-disease-related-covid-19. Although blood biomarkers have been identified I am not aware of blood type playing a role for children.