A sweet biscuit or a piece of chocolate will not harm your dog.
Sweet things are highly palatable for dogs as explained in ‘Feeding your Dog’. However table sugar is one of the disaccharides which are not digested by endogenous enzymes produced in the body of the dog but by the inadequate enzymes available from the epithelial cells of intestines. This results in bacterial overgrowth and sometimes, diarrhea. Further, sugar may cause obesity and dental caries. It is interesting to know that chocolate contains theobromine which is toxic to dogs. Dogs clear theobromine more slowly than human beings. Thus sugar should be avoided for dogs as far as possible.