Original sentence: Lungs fill the blood with oxygen.
Corrected, simple sentence for a 6-year-old: Your lungs put oxygen into your blood.
Why this is better (step by step):
- Say whose lungs: use 'your lungs' so it is clear.
- Use simple words: 'put oxygen into' is easier to understand than 'fill the blood with oxygen'.
- Make the object clear: 'your blood' tells whose blood it is.
What it means (easy): Your lungs take air in. The good part of the air (oxygen) moves into your blood. The blood carries the oxygen to all parts of your body so you can move and play.
Short practice sentence: My lungs put oxygen into my blood.