I raised 5 children, these are tips I found helpful.
Keep nap time early in the afternoon, so they are fully tired for bedtime at night.
Give them a "hardy" meal for supper, full tummy is comforting, but not spicy (upsetting), and restrict nighttime snacks to no sugar. Milk is relaxing, we used to do the malts each night, it actually helped (small glass for young age, yes it's sugar too, but seems the dairy just helped). We're big on hot cocoa here, we cooled it down with cold milk for the younger ones, the "warm" milk is also soothing, our grandmother's used to do warm milk before bed.
A ritual, bedtime story, something to relax them, make them feel safe. NO scary TV, movies and even Disney stories can be scary, nothing to give them dreams that wake them up...we want "good dreams".
Buying something "big girl" special...new bed sheets, new blanket that she picks out no matter how wild it is, and a night light...they have a pretty "light effects" room freshener...and they have plug in room fresheners that are aromatherapy for "relaxing". Soothing sounds machines, or even a CD in a small player softly in her room, keeps her relaxed.
We're often not aware that the children do pick up what's on TV, news, shows...they get scared with fires, wars, guns, bad people, families not together...they dream about it, and even when they awake and are unaware (as often we can't recall the dream a few moments after we awake), it's still what's keeping them from a "comfortable safe" rest.
Hope this helps.