Original sentence: 'With its super nose, an echidna feels a bug moving and grabs it.'
Is it correct? Yes — it is grammatically correct. The subject ('an echidna') goes with the verbs ('feels' and 'grabs'), and the comma after the opening phrase is fine.
Small improvements to make it clearer for a child:
- Say 'the bug' instead of 'it' so it's less confusing: 'With its super nose, an echidna feels a bug moving and grabs the bug.'
- Use simpler, more vivid words: 'With its super nose, an echidna feels a bug wiggle and catches it.' ('wiggle' and 'catches' are easy to picture.)
- Or change the sentence shape: 'An echidna uses its super nose to find a moving bug and catch it.' (Also clear and simple.)
Recommended for a 6-year-old: 'With its super nose, an echidna feels a bug wiggle and catches it.' — clear, correct, and easy to imagine.