Correct sentence (best for a 6-year-old):
When they are in danger, echidnas burrow and show only their spiky backs to stay safe.
Why this is better — step by step:
- When they are in danger tells us when this happens. It is a clear way to start the sentence.
- "Echidnas burrow" is fine — "burrow" already means digging into the ground, so you don’t need "down."
- "Show only their spiky backs" is a bit clearer than "show just their spiky backs." Both are okay, but "only" is easy for young children to understand.
- "To stay safe" explains why they do it. That makes the sentence complete.
Other short options you could use:
- Echidnas hide by burrowing so only their spiky backs show.
- Echidnas dig into the ground and show only their spiky backs to stay safe.
Any of these are correct and easy to understand for a 6-year-old.