Your sentence is mostly clear and correct. Here is your sentence with a small improvement for clarity and flow:
"Jays hide their food in different places. The acorns and hazelnuts they forget about grow into trees, making the forest bigger."
This sentence is grammatically correct and easy to understand. It tells us that jays hide nuts like acorns and hazelnuts in many spots, and the ones they forget can grow into new trees, which helps the forest get bigger.