Our 4 year old has never been a great sleeper. Gave up naps at 2.5, and even without a nap we struggled to get her down before 9 pm, then she was up at 7 am latest. We did the bedtime routine--warm bath, warm milk, story, white noise machine, weighted blanket. Then, she would wake at least once between midnight and morning. She was cranky, and it was slowly killing me and my husband. I hadn't had a full night sleep in years. She finally started sleeping through the night about 3 months ago on her own, but with the 'extra' sleep at night, wasn't going down until closer to 9:30. So, a few weeks ago we started her on a half of one gummy of melatonin (Vicks pure zz's). The dosage on the bottle was 2 gummies, but I wanted to do the lowest effective dose. A half a gummy works great. She's down by 8 pm most nights, sleeps through until 7 am and the attitude change is fantastic. And the whole family dynamic has improved because everyone is getting more sleep. From what I've read, there are two problems with melatonin. The first; since its not a regulated substance by FDA, every time a study is done, the dosages contained in each gummy vary wildly and don't necessarily correspond to what the bottle says. So your child could be getting more of less than the recommended dosage. The second is there aren't any long term studies on impacts of melatonin on children. We're going to keep going with it for a month, and see if that is enough to reset her sleep habits without melatonin. I don't know what the long-term effects are, but the short term effects of a 4 year old not sleeping are, you know, adoption.
https://time.com/6274507/melatonin-sleep-supplement-dosage-off/