Lots of excellent suggestions posted already.
Melatonin works for many; not all though. And doses don’t need to be as high as indicated on the label. Just make sure to run it by your pediatrician.
Never heard of calcium pills promoting sleep. Anti-histamines, yes. Saying Tehillim, yes. Repetitive sound machine, yes. But calcium pills? Very interesting.
mommamia22- your children may not be getting enough energy out of their systems before going to bed. Heavy increase of activity before going to bed might do the trick. I know someone who had this problem with her 2-yr old daughter who actually became more active and agitated as bedtime routine unfolded. So I recommended that her mother have her daughter dance it out, or exercise in some other way, to get her tired and to appreciate the bedtime routine each night. She started taking her daughter out for nighttime walks. It worked like magic! Now her daughter practically begs her to go to bed each night even without the walk and falls asleep almost immediately.