Nice sentence — you have a good idea! Here is a simple corrected version and an easy explanation for a 7‑year‑old.
Corrected sentence: The anthill you see is like a roof for the ant town underground.
- Big letter: Start with a capital letter: "The."
- Right preposition: Use "for" or "over" instead of "to." We say "a roof for" or "a roof over." Example: "An umbrella is like a roof for me."
- Shorter place word: Say "underground" or "under the ground" instead of "that is below the ground." "Underground" is one word and easier to say.
- End with a period: Put a dot at the end of the sentence.
Another simple version: "An anthill is like a roof over an ant town underground."
Try this: change one thing and say the sentence out loud. For example, replace "underground" with "under the ground" to see which you like better.