My 2 1/2 year old was accidentally prescribed omnicef 250mg, 2x a day, but learned it was suppose to be for ceftin (cefuroxime). what harm can this do to the body.

No harm: Both medications are cephalosporins, and should treat similar infections. The dose may need to be adjusted though according to the weight of your child.

Possibleside effects: The antibiotic at ~ 5x it's recommended dose just increases the risk of diarrhea & vomiting sometimes associated with the medication.As an antibiotic it works fine at proper doses.There is no specific toxic effect like you would see with excess tylenol (acetaminophen) or other medications.