Executing Child packages without using master package
I use the following as the guideline:

If you are simply using many packages to structure your solution in a more modular fashion, executing in-process is probably the way to go because you don't have the overhead of launching more processes. I'm happy to pass on that advice here.

Quoted from: http://consultingblogs.emc.com/jamiethomson/archive/2005/05/16/1414.aspx