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Not to worry. There is not enough chocolate in mild chocolate to cause toxicity in these dogs (especially with such a small quantity of milk chocolate). Also, the aluminum foil is not likely to cause a foreign body reaction.
I've seen dogs that have done this dozens of times and never had a serious problem. It is possible that they might have some vomiting (but I doubt it) or diarrhea (more likely). Either of these should be self limiting. But, if the diarrhea persists then you should withold food for 24 hours, then feed a bland diet (boiled hamburg or chicken with fat removed mixed with equal amounts of boiled rice) for a few days, then gradually, add regualar food back into diet.
If vomiting persists, especially with lack of appetite, then a trip to the vet is essential in the very unlikely case that an intestinal blockage has occured.
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No doctor patient relationship is formed by this communication. This advice is no substitute for a complete physical examination and appropriate diagnostics provided by a licensed veterinarian.