Nice sentence! Here is a clear, correct version for a 6-year-old:
After the autumn rains, the forest is full of mushrooms.
Why this is better:
- When a sentence starts with words that tell when something happens (like "After the autumn rains"), we usually put a comma after that beginning.
- Adding the word "the" before "autumn rains" makes the sentence sound natural: "the autumn rains."
Another okay version is: After autumn rains, the forest is full of mushrooms.