Checking for existence of parent variable in child package

I have a parent package that calls a child package, when I run the parent package the child package picks up a variable value from the parent in a script task and runs fine, the problem I'm facing is when I run just the child package, the script task fails because it doesn't know about the parent variable. The dilemma I'm facing is in my child script task, if I add the parent variable to the ReadOnlyVariables list then the task fails because the parent variable doesn't exist when I just run the child. If I don't add the parent variable to the ReadOnlyVariabls list then if I try to use it then the task fails saying that the variable doesn't exist in the variables collection.

Is there a way to check for the existence of the parent variable, so when I just run the child package I don't get an error and I don't have to change my task every time I choose to run the child package only vs running the parent/child

Thanks.

Basically this is what I have:

Parent Package

Variable: P_SCHEDULE_JOB_ID

Child Package

Variable: SCHEDULE_JOB_ID

Script Task:

ReadOnlyVariables: P_SCHEDULE_JOB_ID

ReadWriteVariables: SCHEDULE_JOB_ID

Dts.Variables("SCHEDULE_JOB_ID").Value = Dts.Variables("P_SCHEDULE_JOB_ID).Value.ToString()

Which works fine when I run the parent package and it calls the child package. When I run only the child package it has no knowledge of the parent package variable and causes the task to fail.

That worked, thank you for your help.