Here is a little supplication/prayer/dua that is easy to teach to little ones, especially if they observe you saying it. Have them repeat after you as soon as they are able to mumble words. Then kiss them and tuck them in tight.
For older children who can read, help them to memorize it. Print this dua for sleep printable [pdf]. Attach to a wall near the bed so the children can see it and read it before going to sleep.
Bismikal-lahumma amutu wa ahya
‘In Your name O Allaah, I live and die (expire and return to life).’
Narrated Abu Dhar
Bukhari Vol 8, Book 75, No 337
Arabic image from makedua.com
Clipart from WPClipart.com