I used to see the error "Errors exist for one or more children." a lot in BizTalk 2004 when I was creating an orchestration. The error wasn't valid, because I could usually get rid of it by saving and then deleting code from an Expression shape, then compiling, then copying the code back, and compiling again.
I had not yet encountered this issue with BizTalk 2006 until today. This time the error was tagged to my Loop shape. I was able to fix the error by deleting the code from an Expression shape inside of the loop, and proceeding as I describe above. I'm just glad that I didn't have a large number of expression shapes inside of my Loop, because there was no way for me to identify which Expression was causing the error.