Good try! The sentence just needs a capital letter at the start and a period at the end. A correct version is:
The lungs fill the blood with oxygen.
A simpler way to say it that might be easier for a 5-year-old to understand is:
The lungs put oxygen into the blood.
Or even shorter:
When you breathe, your lungs give oxygen to your blood.
Quick writing tips: start the sentence with a big (capital) letter and end it with a period. Practice by saying the sentence out loud and then writing it down the same way.