**Core Concept**
Decreased serum sodium can lead to seizures in children due to the osmotic imbalance it creates in the brain. This condition is known as hyponatremic encephalopathy.

**Why the Correct Answer is Right**
When serum sodium levels drop, water rushes into brain cells, causing them to swell. This swelling can lead to increased intracranial pressure and potentially life-threatening seizures. In pediatric patients, the brain is particularly sensitive to changes in serum sodium levels, making it more susceptible to seizures. This is because the pediatric brain is still developing and has not yet fully adapted to regulate its water balance.

**Why Each Wrong Option is Incorrect**
**Option A:** Decreased serum magnesium can also lead to seizures, but it is less common in children and typically occurs in the context of severe magnesium deficiency or hypokalemia.
**Option B:** Decreased serum potassium can lead to muscle weakness and cardiac arrhythmias but is not typically associated with seizures in children.
**Option C:** This option is a duplicate of Option C, which is incorrect, so it is also incorrect.

**Clinical Pearl / High-Yield Fact**
It is essential to note that seizures in children can also be caused by other factors such as fever, infection, or metabolic disorders. A thorough evaluation, including laboratory tests and imaging studies, is necessary to determine the underlying cause of seizures in pediatric patients.

✓ Correct Answer: A. Decreased serum sodium.