how do I change the properties of a material that are on the children of an object on script?

This is the code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class TargetBehaviour : MonoBehaviour
 {
     [SerializeField] GameObject shatteredTarget;
     [SerializeField] float breakForce;
     [SerializeField] float durationDissolve = 1f;
     GameObject frac;
 
     public void DestroyTarget(Vector3 hitPos)
     {
         frac = Instantiate(shatteredTarget, transform.position, transform.rotation);
 
 
         foreach (Rigidbody rb in frac.GetComponentsInChildren<Rigidbody>())
         {
             Vector3 force = (rb.transform.position - transform.position).normalized * breakForce;
             rb.AddForceAtPosition(force, hitPos, ForceMode.Impulse);
             Debug.Log("Force applied");
         }
 
         StartCoroutine(CutOffEffect());
 
         Destroy(transform.parent.gameObject);
         Destroy(gameObject);
 
     }
 
     IEnumerator CutOffEffect()
     {
             foreach (Renderer fracRenderer in frac.GetComponentInChildren<Renderer>())
             {
                 float timePercent = 0f;
                 Debug.Log("Dissolve effect starting");
                 while (timePercent < 1)
                 {
                     timePercent += Time.deltaTime / durationDissolve;
                     fracRenderer.material.SetFloat("_Cutoff", Mathf.Lerp(0f, 1f, timePercent));
                     yield return null;
                 }
 
             }
         
     }
 
     
 }

On the IEnumerator CutOffEffect i'm trying to change the float of a property "_Cutoff" in every children's material that are on "frac".

 foreach (Renderer fracRenderer in frac.GetComponentInChildren<Renderer>())
             {
                 float timePercent = 0f;
                 Debug.Log("Dissolve effect starting");
                 while (timePercent < 1)
                 {
                     timePercent += Time.deltaTime / durationDissolve;
                     fracRenderer.material.SetFloat("_Cutoff", Mathf.Lerp(0f, 1f, timePercent));
                     yield return null;
                 }
 
             }

But on this foreach i get an error, i don't why.