There’s no cognitive dissonance here, and far too many people misuse the term. Cognitive dissonance is a feeling of discomfort a person feels from holding beliefs that contradict each other or from behaving contrary to their beliefs.

This is just plain old stupidity. These assholes believed vaccines are a hoax, their daughter died as a result, and they still believe vaccines are a hoax. They behaved exactly as one would expect antivaxxers to behave.