I'm still trying the execute package solution rather than moved on to running it in dtexec. It fails in BIDS, but I have a sql agent job that runs the deployed package and that's stopped working now as well, which is a bit more critical as the team I support rely on that one. The SQL Agent job runs as a different user but uses the same dsn connection setup.

I can test the odbc dsn connection in admin tools data sources (ODBC) and it connects. I also set up a test package in BIDS to connect to the same data source and that connects fine, so it's a mystery why my other two packages don't work (child on its own, and parent running child).