Game object inheriting children colliders

I have a script in my game, where the player checks when it collides with the ground and then sets "isGrounded" true, and sets it false when the player leaves collision with the ground-- the script allows for limiting jumping while in midair.

Anyhow, it works when I have a single GameObject plane that I reference-- but I tried collecting all my surfaces into an empty game object and checking for collision with that. It doesn't seem to work. What I'm trying to do it get a parent game object who's collision shape is the sum of all it's children's shape. Here's my code:

 public GameObject floor;

 public void jump()
 {
     if(isGrounded)
         rigidbody.AddForce(new Vector3(0,300,0));
 }


 void OnCollisionEnter(Collision c)
 {
     if(c.gameObject == floor)
     {
         isGrounded = true;
     }
 }

 void OnCollisionExit (Collision c)
 {
     if(c.gameObject == floor)
     {
         isGrounded = false;
     }
 }
 

And then I plug my parent-game object (of all the surface area I want to count as the floor) into the script, and it does not register the collision handlers.

How could I have the parent game object have the same collision box as all it's children? Or is there a better way to do this? Thank you in advance.