GTP-4 Chattie has three settings, "more creative," "more balanced," and "more precise." I just submitted your prompt on the "more precise setting" and got this:

There is no evidence to suggest that mRNA vaccines cause cancer in children. In fact, mRNA vaccines are being researched as a potential treatment for cancer123. The mRNA used in vaccines does not alter DNA and therefore cannot cause cancer45.

So it begins with "there is not evidence to suggest" which somewhat addresses your complaint that it made an absolute statement of fact to you. What setting did yo have it on?