Simple and clear grammar correction of a sentence describing stinging sea anemones with a happy family of clownfish for young learners.
Your original sentence is:
Show stinging sea anemones harbouring a happy family of clownfish.
To make it grammatically correct and a bit clearer, you can say:
"Show stinging sea anemones harboring a happy family of clownfish."
Note that 'harboring' (American English) means 'holding or having'. Also, if you prefer British English, 'harbouring' is fine.
So a clear sentence for a 6-year-old could be:
"Look at the stinging sea anemones holding a happy family of clownfish."
This simple sentence shows that the sea anemones have the clownfish with them.