Don't check the 'Continue building' step option of the Exit step (I guess it could be considered a bug that an Exit step configured in this way actually exits the build [rather than continuing], but it doesn't sound like that would help your situation anyway). You can still debug the child build instance without using that option (just close the child instance after the build stops at the failure).
If you sometimes want to debug the child project but other times want the child build to exit on build failure, use a field override (triggered from a temporary macro, for instance) for the 'silent mode' option.
http://www.visualbuild.com/Manual/ac...ldoverride.htm