Your sentence is mostly correct but can be improved slightly for clarity and grammar. Here's the corrected version:
"Up in the mountains, a fluffy chinchilla basks in the sun, a chamois hops from rock to rock, and a condor soars high above. Mallards flock together to fly to warm countries."
Explanation of changes:
- Added a comma before "and a condor soars high above" to correctly connect the items in the list.
- Everything else is well-written, and the words are easy to understand for a 6-year-old.
Great job describing animals and their actions in the mountains!