L.M.
Have you tried Calms Forte for Kids? It's a homeopathic for sleep that you can get at the health food store or online. It is very natural and totally safe and in a sugar base, so I call it "candy". I like the melatonin idea too.
I have no idea if this applies to you, but my 18 mos old wasn't sleeping well and it turns out he is allergic to the fibers in this stuffed animal he can't live without and this one natural doc helped me with that and now he is sleeping great. Or she could have a food sensitivity that a naturopath could diagnose. Just ideas... Good luck.