I agreed with Alchemist, detailed answers to your question. 2-5 mmol/kg/day is the revised recommendation for maintenance for neonate and pediatrics. The dose can be lower or higher than the recommendation and it based on underlying problem. Nora listed the common etiologies for hyponatremia.
Also, you have to consider when there is a situation of high Na requirement, dilutional effect like what you see in CHF, cirrhosis, severe hypoalbunemia that need to minimize fluid intake. Also, you have to pay attention to false hyponatremia like in case of uncontrolled blood glucose, hyperlipidemia, etc.
I advise you if the baby required high sodium supplement to do electrolyte profile at least 2-3 times per day to avoid over correction
in addition to urinary Sodium level