I am triggering one job from another job. Now the child job having 2 prompt variables which is prompting while triggering the parent job, but value is not passing to the child job. It is taking the default value defined in the child job only.

I am guessing you are using the trigger build step. If so, the step can define variables passed to the build being triggered.

Also is there any concept of global variable in QB. If yes, then how to define it can you please suggest?

Variables defined in root configuration can be seen by all configurations.