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Yes. Both are grammatically correct. The first sentence is complete and sounds fine.
However, on its own as a complete idea, the second sentence seems odd. The listener would think that you were going to say more. You have emphasized that it is the children who are fishing. Why are you pointing that out...there must be more coming.
So, for example:
There are 3 children who are fishing, and 2 adults sitting by a campfire.
Fishing 
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