No! A single tonic-clonic seizure does not cause mental retardation or brain damage! Nor do recurrent seizures cause mental retardation or brain damage. It is true that tonic-clonic and other seizures occur more often among children who already have brain damage, learning problems, or mental retardation, but there is no evidence that seizures make these conditions worse. Most children with such problems do not have seizures, and most children who have seizures do not have these problems.
Of course you are afraid that seizures will recur. But don’t let this anxiety control your life or your child’s life. Don’t allow it to make you so overprotective that your child can’t play or go outside without your constant supervision. If no new seizure comes, think of the single seizure like a fall out of a tree, something frightening at the time but over and not influencing your life or your child’s life forever.
If seizures do recur, most likely they will recur within two to three months after the first seizure. It might be reasonable to be a bit more cautious during this brief period. Maybe your child shouldn’t be climbing trees or be in other potentially dangerous situations during this time. But there’s no reason why he can’t swim, play most sports, go on field trips, and lead an otherwise normal life.