Someone else filed a return using the child's SSN. Accidentally or on purpose.

I filed the child's return and hit the correct boxes that the child is being claimed by the parents. 

Then someone else has filed a return in that child's SSN.   You'll be filing the parents by paper.

If the child is a teenager, he may have used a phone app "just to see what his refund would be" and ended up filing it himself.

I never file a dependents return before I file the parents.  I am always afraid, despite double checking, that I hit a wrong box somewhere.  I would much rather deal with an issue with the dependents than with the parents.

I flubbed it up one time, now Im paranoid as well, always file the parents first too, then I know Im not the one that  screwed up!