Sentence correction for a 6-year-old: Light and fast, the deer easily make their way through the thick forest

Correct (many deer): Light and fast, the deer easily make their way through the thick forest.

Or (one deer): Light and fast, a deer easily makes its way through the thick forest.

1. Don't use all caps: Write deer, not DEER. All caps looks like shouting.
2. Singular vs. plural: If you mean more than one deer, say the deer and use make and their. If you mean one deer, say a deer and use makes and its.
3. Comma is fine: The comma after "Light and fast" is good because it's a short describer at the start of the sentence.
4. "Thick forest" is good: That phrase describes the forest well.

Great job — choose the plural or singular version depending on whether you mean one deer or many!