Great question.  Most experts agree that the adult dose of 1.25 mg xopenex es equally safe for kids.  I think the main reason for this is because the airways of kids is much smaller than the adult airway and so disposition of the medicine into the lungs is qreatly diminished. 

Likewise, the most common method of delivery of the medicine to neonates is by blowing the med by the patient's face (blowby treatment), and this results in most of the medicine being wasted to the atmosphere (perhaps even as much as 80% of the medicine wasted). 

In fact, according to respiratory therapy experts (read this article) the use of the blowby is not recommended.  However, with neonates, we have no real other options.  Therefore, the 1.25 mg dose is recommended. 

A preferred method to blowby is using a face mask, but still 50% of the medicine is wasted to the atmosphere.  The best method is using a mouthpiece, but that's not possible with the neonate population. 

However, despite what I have written here, the best dose of xopenex is whatever dose works best for your child with the least side effects, and whatever dose your doctor recommends.  Most doctors where I work prescribe the 1.25 mg dose, however, on occasion, they recommend the lower doses. 

I hope this helps.  Any further questions let us know.