A very frustrating situation, I know. My youngest had this problem, no matter what I did.
Remaining consistent was the key. A set bed time. Dimming the lights, and a warm bath, with lavender, helped. No stimulating TV. Mash worked with my oldest. When the theme song came on he knew it was time for bed. He used to try and change the channel on me, of course I laughed. Nice try.
With my youngest I had to eliminate red dyes. Had to make sure he had no juice containing red dye. It made him nuts. There are numerous people who are allergic to yellow dye, and blue lake dye. Allergies are not limited to just a runny nose, or sneezing. Just depends on the person. Keep track of what they eat or drink, and notice their response. It might give you a clue to what might be keeping them from sleeping. They will react the same during the day. Excess sugar close to bedtime, as well as caffiene will do the same thing. A glass of warm milk not only contains calcium, it also contains trytophan. An amino acid that induces calm, and sleep. Nut butters, whole grains, and turkey contain trytophan as well **Please note**Do not give children or teenagers amino acids, in pure form. Better to derive from food.
Calcium, and magnesium are very relaxing to the body, and mind. Usually taken an hour before bedtime. Works for adults too. Helps with leg cramps. Of course as an adult you would take an adult formula, but for children they make a childs dose:
The only side effect of magnesium is diarrhea. The active ingredient in Milk of magnesia. If this occurs just reduce the dose.
I hope it helps:)