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Hello and thank you for using this site to help you. If he ate apples including the core then it may cause an obstruction of the bowel. If he ate apple seeds then then they contain cyanide and can make a dog ill.
Has the dog eaten anything for you? Has there been any vomiting or diarrhea?
Here is more on apple seed toxicity http://www.aspca.org/pet-care/animal-poison-control/toxic-and-non-toxic-plants/apple
I would certainly let the day care know that it is negligent to allow the dogs to eat apples or allow them access to the field.