What side effects occur in a child if he was overdosed with ascorbic acid during childhood?

Probably none: While overdosing any vitamin is never recommended, vitamin c is readily excreted in the urine so any excess Ascorbic Acid (vitamin c) doesn't usually build up in the body or cause long-term problems.

Mineral deficiencies: Mega dose vitamin c can actually be dangerous. It is true that vitamin c is water soluble & you will excrete any vitamin c that you don't use. However, in order to excrete vitamin c in your urine, you must also excrete minerals as well. Therefore, excess vitamin c can lead to mineral deficiencies, especially a copper deficiency. Be very cautious in megadosing any vitamin or mineral or amino acid.

Lots!: He could get belly pain, heartburn, nausea, vomiting, diarrhea, painful urination, bloody urine, headache, flushing, lightheadedness, kidney stones, or even an allergic reaction (hives, facial swelling, etc.). Also Ascorbic Acid (vitamin c) can interact or interfere with medications. Talk to his doctor about your concerns, and have him seen right away if he's having symptoms.