Original sentence: An echidna’s nose can detect any bug when it’s on the move.
Corrected sentence: An echidna’s nose can detect any bug that is moving.
Even simpler: Echidnas can smell bugs that are moving.
Why this is better: The phrase "when it’s on the move" is a bit long and unclear. Saying "that is moving" or just "moving" clearly describes the bug. For a 5-year-old: "on the move" means the same as "moving," so we use the shorter word to make the sentence easier to understand.