Remove global variables from Child package, child will use the parent variables if you refer inside if Child which are declared in Parent and assigned values either in parent /child.
Same approach in DTS / SSIS
Parent : declare one global variable
add script task, assign some value to global variable
add execute package, call child package
Child : DON't declare any variable
Create script task, print (msgbox) parent variable
execute parent, you will see parent variable value in child.
Do not select inner/outer variables for this above example...