Original: The echidna's nose feels the tiniest wiggles, so it knows where the bugs are.
Corrected (simple for a 3‑year‑old): An echidna's nose can feel tiny wiggles, so it knows where the bugs are.
Why: 'An' sounds natural before 'echidna.' 'Can feel' shows it is able to feel. 'Tiny' is simpler than 'tiniest' for young children.