I could not build a BT project because of an error reported in an orchestration: “Errors exist for one or more children.”.  The orchestration was added as an existing item since I was changing some schema namespace references.  An expression shape had the red exclamation to show this was the issue (or not).  The expression was one line where a string variable was being populated by a distinguished property.  It looked like the distinguished property was the issue, but I tried many things to no avail: rebuilding the schema project, re-referencing, removing the expression, building and then putting back etc…  The only thing that worked for me was some advice I found here: http://thenewblackisnowback.blogspot.se/2012/06/biztalk-errors-exist-for-one-or-more.html
Number 3 was what helped me:

3) Look in the odx file as text and search for the error there(“Errors exist for one or more children.”) sometimes it will be near the bottom. Delete it and try to rerun.