SSIS: Passing variable from child package to parent package
This can be done using a Script Task in the child package to update a variable from the parent package based off the value of a variable in the child package, assuming that none of the variables that are accessed have the same name in both the parent and child packages.  While the variables from the parent package won't be displayed in the ReadWriteVariables field on the Script Task, if you enter them here then they can be accessed.  Updating the variables from the parent package would follow the same syntax as doing so for variables in the same package, and the C# example details this.

Dts.Variables["User::ParentVariable"].Value = Dts.Variables["User::ChildVariable"].Value;