The sentence you wrote is almost correct, but we can make it clearer and more grammatically accurate. Here's a better version:
Correct sentence: "Mice use their little holes to store grains and hide from cats."
This sentence is grammatically correct. However, if you want it to be simpler for a 4-year-old, you could say:
"Mice hide in their little holes and keep grains there."
Both versions convey the meaning clearly and are good for young learners.