Trigger in children object activates function in parent

My prefab is a sphere with an empty game object (that has a trigger) attached to it. It is supposed to represent a bomb.

The way it is supposed to work: The prefab is thrown at an enemy. When the "bomb" makes contact with another gameObject that has the tag enemy it sends a message to the empty game object. Then, the empty object destroys all the gameObjects with the "enemy" tag within its trigger.

The way it works: The prefab is thrown at an enemy. When an enemy touches the trigger of the empty object the message that destroys the gameObjects is sent.

I recently updated my project from unity 3 to 4. Everything was working correctly before the update. I haven't changed anything in the code but my object behaves differently, now.

The script from the sphere:

 public class BombCollision : MonoBehaviour {
     
     // When an enemy hits the bomb, a message is sent to the child object that has the tigger attached
     void OnTriggerEnter (Collider collider) {
         // Debug.Log ("Hit something that is not an enemy");
         if (collider.gameObject.tag == "Enemy") {
             // Debug.Log ("Hit the bomb");
             BroadcastMessage("EngageTargets");
             StartCoroutine("timeForExplision");
         }
     }
     
     // gives time for BombKiller to destroy the objects are within the collider before destroying the bomb
     IEnumerator timeForExplision () {
         yield return new WaitForSeconds(0.05f);    
         Destroy(gameObject);
     }

The script from the empty game object attached to the sphere:

 public class BombKiller : MonoBehaviour {
     
     bool contact = false;
     
     void OnTriggerStay (Collider collision) {
         if (collision.gameObject.tag == "Enemy" && contact == true) {
             collision.SendMessage("WasHit");
         }
     }
          
     void EngageTargets () {
         Debug.Log ("got engageTargets");
         contact = true;    
     }
 }

I've tried to solve this for a whole day now, I hope someone can help me out, thanks for reading.