It is not true that they are immune, but it does seems that they don't have very mild or no symptoms. Twitter is being pedantic here.
“Isn’t this true? How many children are getting COVID? How many have died?”
I mainly follow Colorado. There have been about 1,850 deaths from COVID in Colorado. For the age group 0-9 years old, deaths are 0. For the age group 10-19 years old, deaths are 3.