Masking mesh of all child objects based on parent object mesh

Hello Everyone,

I am not sure if this is possible to execute but i want to mask all the mesh in the child object based on the Parent objects mesh.

Here is a better picture for what i am trying to achieve.

The two red cubes are set as the child of the big Green rectangle cube. Is there a way in which i can mask the portion lying outside the green rectangle making only the portion within the green rectangle visible?

Thanks in advance