How about: "Have you ever been frustrated by your computer / phone / smart-assistant when you wanted it to do thing A, and instead it did thing B?
Sometimes that is because someone other than you wants B and has more control over your device than you do.
Information Security is about making sure that your device does what you want it to do, instead of what someone else wants it to do."
It doesn't focus on the information part, but fundamentally security is about control, and elementary students have probably already been frustrated by computers and probably already have a sense of ownership == control.