The error “Errors exist for one or more children” can often be tricky to uncover.
Usually, when I get this error, I double click on it, and an orchestration shape will be slightly highlighted or bolder than normal, and it will have a red exclamation point on it. You then open it, look for a tiny red squiggly mark, and try to guess what the syntax error is.
Today, I got this error, but no orchestration shape was highlighted. I was about to pull my hair out, then luckily, out of the corner of my eye, I saw a red exclamation mark in the Orchestration Viewer next to a Multipart Message. Apparently I had added a new one, and either 1) not connected it to any schemas, or 2) not deleted it.
Moral of the story – look in the Orchestration Viewer, not just the orchestration shapes for the error.