Before a sequel, why not an origin story? Roald Dahl wrote a couple of WWII themed books.
The second one's not for children:About mischievous little creatures that were part of Royal Air Force folklore. The RAF pilots blamed the gremlins for all the problems with the aircraft. While at the British Embassy in Washington, Dahl sent a copy to the First Lady Eleanor Roosevelt who read it to her grandchildren, and the book was commissioned by Walt Disney for a film that was never made.
They briefly emerge during the Battle of Britain, but the experience makes them decide that humankind would destroy themselves without their intervention. The gremlins wait until the time is right - after World War III and World War IV decimate the earth, they emerge and take over the world. The story ends with the gremlins, unable to exist in a world without humans, disappearing too; finally, only the worm remains.