Let's look at your sentence step by step:
Original sentence: "The jungle is a forest that gets lots of sunshine and rain. This helps plants grow all year round!"
This sentence is grammatically correct! It clearly tells us that jungles get lots of sunshine and rain, which helps plants grow all year.
One small suggestion: you can add a hyphen in "all year round" to make it "all-year-round" if you want it to be an adjective. But as it is, it's perfectly fine.
So, your sentence is great for explaining the jungle to a 7-year-old!