Welcome Sharon,
I applaud your willingness to teach the nominative absolute to sixth graders!
You may, however, find that it's a bit sophisticated for them. I suggest that
you refer to this structure as a noun plus a participle (ing or ed word). Try
sticking with the animal examples:

The raccoon, its nose buried in the garbage, ignored us when we opened the
garage door.
Feathers fluttering, the canary flew out of its cage.

Good luck, and may I recommend Harry Noden's Image Grammar: Using Grammatical
Structures to Teach Writing, published by Heinemann.
Amy Benjamin

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