Many stories begin with a dangerous object: "Where's Papa going with that axe?" (Charlotte's Web by EB White). Another way is to introduce tension by denial: "I don't scare easily." (The Phantom Thief by Pete Johnson). A setting with an ominous element opens How Hedley Hopkins Did a Dare by Paul Jennings: "They say there is something awful in the sand dunes." We can tell already that the main character will find out what it is soon enough.
Another type of hook places the main character directly in danger: "I disappeared on the night before my 12th birthday." (Kensuke's Kingdom by Michael Morpurgo). Collect and list examples in the children's writing journals and use them as a basis for creating their own hooks, for example:
"Sanjay didn't believe in ghosts."