All three sentences are grammatically correct. Here they are neatly written:
Marmots live in burrows. They warn each other of danger with loud whistles. It's not easy to catch them unaware.
Step-by-step, easy explanations for a 7-year-old:
- "Marmots live in burrows." — This tells where marmots live. "Marmots" is the name of the animals, "live" is the action, and "burrows" is the place. Everything is in the right order.
- "They warn each other of danger with loud whistles." — "They" means the marmots. "Warn each other" means they tell one another. "Of danger" says what they warn about, and "with loud whistles" says how they warn. The sentence is clear and correct.
- "It's not easy to catch them unaware." — "It's" is short for "It is." This means marmots are hard to surprise. The phrase "catch them unaware" is a common way to say "surprise them," and it is correct here.
Small alternative you can use if you want a different wording:
Marmots live in burrows, and they warn one another of danger with loud whistles, so it's not easy to catch them off guard.
If you want, I can help you make the sentences longer, shorter, or more exciting for a story.