The sentence you wrote is mostly correct but can be made a little clearer and smoother. Here's the corrected version:
"An oasis is like a green island in a sea of sand. It appears in places where water comes out of the ground."
Explanation:
- Use "An oasis" at the beginning because 'oasis' starts with a vowel sound.
- Change "the sea of sand" to "a sea of sand" because we are talking about any sea of sand, not a specific one.
- The rest of the sentence is good and easy to understand.
This makes your sentence grammatically correct and still simple enough for a 6-year-old to understand.