Original sentence: Animals stash food in burrows, tree hollows or in the ground.
Corrected sentence (clearer grammar): Animals stash food in burrows, in tree hollows, or in the ground.
Why this is better: Repeating in before each place makes the sentence balanced (we call this "parallel structure"). It sounds smoother and is grammatically correct.
Simple version for a 6-year-old: Some animals hide food in burrows, tree holes, or in the ground.