Corrected sentences:
Tundra trees are dwarf trees. They hide from the frost and wind by staying close to the ground.
Why this is better:
- Adding "by" shows how the trees hide (it explains the action).
- The sentences are short and clear for a 7-year-old reader.
- You can also combine them into one sentence: "Tundra trees are dwarf trees that hide from the frost and wind by staying close to the ground."