What is the possibility of organ damage in a 12 year old due to overdose of 600 mg of atenolol with out treatmeant.?

The possibility?: Anything can happen in life, so one must not ask, "what is the possibility?" the real question is, "how likely is it?" if a person took a several times the normal dose of atenolol, got evaluated by the doctors at the hospital, and was found to be recovered back to normal condition, then the answer is "unlikely". The primary care doctor can recheck if desired or if needed.