Could the child have picked up casino nicotine/grime from the ball on the pull handle and from the pushbuttons or other surfaces of the machine? All the years in a casino can put a lot of residue on things. Kids love to touch everything and some kids are more sensitive to stuff than adults are. In the old days people playing with coins sometimes wore thin cotton gloves, mainly to keep their hands clean from handling the coins but it also may have helped with allergies, although when you were sitting in a smoky casino gloves didn't do anything for that. Even though the machine is in a clean home it could still transfer some old nicotine crud to the little hands. You can wipe down the cabinet with a white paper towel and some household cleaner and see if the paper towel shows anything. If there is a yellowish stain that's most likely nicotine.