Here is a clear, corrected version of the sentence for a 7-year-old:
Original: In summer, Arctic foxes are brown to blend with the rocky soil.
Corrected: In summer, Arctic foxes are brown to blend with the rocky soil.
Note: The original sentence is almost correct. If you want to make the purpose even clearer, you could say:
- In summer, Arctic foxes are brown to blend with the rocky soil.
- In summer, Arctic foxes turn brown to blend in with the rocky soil.
Tips for checking grammar:
- Make sure the subject (Arctic foxes) and the verb (are) agree in number.
- Use "to blend" to explain why they are brown.
- Use "in" with "blend in with" when you want to say something fits in with its surroundings.