It's called Proposition 65. Otherwise known as the Safe Drinking Water and Toxic Enforcement Act of 1986. California voters wanted to be warned if anything has any of 750 chemicals that are known toxins in them. Many of these toxins continue to be used all over. Only California issues that warning.
You can read all about it here.
http://www.oehha.ca.gov/prop65/background/p65plain.html
I wouldn't sweat it too much. Anything with aspirin, for instance would get a birth-defect warning on it in California. Hot dogs, french fries and potato chips have known cancer-causing substances in them, so all of Disneyland's food, for instance, is carcinogenic.
But Disney probably has that warning on all its receipts. If there's no * next to any of the purchased items, then your parents didn't get anything with one of those chemicals.