My stock standard trick for these sorts of problems are mapping drives.
Either by using a mapped network drive or by using Subst (both methods are interchangable).
e.g. Map the location of your package to N:\ then inside your package use paths using N:\MyParentPackage.dtsx, N:\MyChildPackage.dtsx. The packages can be on totally different drives in different folders on different machines, it'll work once you map the package location to the N:\
I usually put a script along side the project files to map the drive, which maps the drive so it can be easily run before. One gotcha, if you're using subst on VISTA - Win8, map it for elevated and non-elevated.
I use the same approach for file references in Visual Studio projects. Only issue with this approach, you use to solve too many issues in your dev environment and you'll run out of drives letters.