Corrected version (for a 7-year-old):
Your heart is a strong muscle about the size of your fist. It pumps blood through twisty tubes called blood vessels. This keeps you alive.
What I fixed and why:
- Changed "Our" to "Your" so the sentence talks directly to the child.
- Kept the sentences short and simple, which is easier for a 7-year-old to understand.
- The original punctuation and grammar were already correct, so only the pronoun was changed for clarity.
Optional one-sentence version:
Your heart is a strong muscle about the size of your fist, and it pumps blood through twisty tubes called blood vessels to keep you alive.