When defining the connection for an Execute Package Task, there's no apparent way to specify the package's (i.e., child's) existence in an *SSIS Package Store* (not MSDB or File System) in a manner similar to what we'd do with DTEXEC and a Master pkg: "\File System\Child".
Sure, one could build an expression that produces a fully qualified path to where the SSIS Package Store typically points to, but that defeats the flexibility offered by msdtssrvr.ini.xml. It would be nice to 1) change the registry on remote servers to point to this XML file on a central server, and 2) use the XML file's ability to redirect SSIS Package Store locations. The payoff would be if my UNC-like reference to "\File System\Child" in the Execute Pkg Task connection manager was automatically insulated from such redirects!! Again, remote servers would point to the XML via their registry setting; the XML would point to package locations that could be changed as desired from our central XML file; our connections wouldn't be the wiser!
Master > Child pkgs are great. So is the SSIS Package Store. It's a crime they don't work together.