Your sentence is almost correct but can be improved slightly for clarity and grammar. Here's the corrected version:
"Wind picks up sand and carries it to new places. That’s how it makes sand hills called dunes and barchans."
Explanation:
- Removed the word "And" at the start of the second sentence because starting with "And" is less formal and can be omitted here for clarity.
- Removed the comma before "called dunes and barchans" because it is not necessary.
This version is clear and easy for a 6-year-old to understand.