First, you should be careful when using Global rph in pediatrics because most of its data are adult patients’ related.

I would go with Lexi:

IV: Dilute in a compatible solution (eg, D5W, NS) to a usual concentration of 0.5 mEq/mL of elemental magnesium (ie, 60 mg/mL of magnesium sulfate); maximum concentration: 20% (ie, 200 mg/mL of magnesium sulfate or 1.6 mEq/mL of elemental magnesium).

Do not freeze. Refrigeration of solution may result in precipitation or crystallization.

Do not recommend to add to TPN as may result in precipitation.

I would not keep the prepared solution for more than 24 hr, especially if prepared outside a sterile area.