Please be careful that you are giving a physiological dose (making up for deficiency only). From what I can reason from years of trying to solve sleep issues, the action required for melatonin to be effective in keeping one asleep is a state of lowered cortisol. This is a healthy function, but if it's brought about with an overdose of melatonin, it could cause breathing difficulties in susceptible children, such as those with a tendency toward croup or asthma.
During croup or asthma attacks, a cortisol-type synthetic hormone is given to open the airways.
Try supplementing with magnesium citrate before going to melatonin. We dropped melatonin when our son needed a 1am, emergency dose of Ativan so he could breathe.
It seems that what we all needed was supplementary magnesium. When the peas are still being harvested, we can eat those and have plenty, but when they're done, we need capsules. Also, cod liver oil (with the right proportion of A and D) plays a vital role in absorption.
Next year, we'll quadruple our pea crop for this reason. Mag citrate and CLO solved the sleep problems, which were severe for all of us. I wish I'd come across this earlier.
Now instead of suggesting looking further into hormonal issues, I highly recommend researching the role of magnesium in the body so you can find out where along the pathway things are going wrong.