Ate marshmallows roasted w/ fake fire log. company says 'non toxic waxes' "california proposition 65... by-products known to cause cancer." concern?

No: Please forgive my frankness. A dysfunctional tort law system has developed in the usa (california's worst) and requires the disclosure of trivial / non-existent risks to prevent. Some products of combustion are known to cause cancer (smoking) & anything can be made to cause cancer in ridiculous doses in animals. It's so you don't sue if you roast a marshmellow & get cancer 30 years later.

Once?: If you did this once, it is probably not a problem. But burning those fake fire logs in the house often may increase your risk. There are a lot of other ways to decrease your cancer risk, so this is probably not a big deal. Eat healthy, avoid stress, get plenty of rest. These are your best bets for decreasing your risk of cancer.