Custom Editor to show child component on parent?

There you go:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
     
       
 [CustomEditor(typeof(TestClass))]
 public class TestClassEditor: Editor {
     
 private TestClass myTestClassInstance;
         
 void OnEnable(){
     myTestClassInstance = (TestClass) target;
 }
     
 public override void OnInspectorGUI () {
         DrawDefaultInspector();
             
     BoxCollider myBoxCol = myTestClassInstance.GetComponentInChildren<BoxCollider>();
         
     if(myBoxCol != null){
         SerializedObject boxSO = new SerializedObject(myBoxCol);
         if(boxSO != null){
         SerializedProperty prop = boxSO.GetIterator();
         while (prop.NextVisible(true)) {
               if(prop.name != "m_Script" && prop.name.StartsWith("m_")){
                          EditorGUILayout.PropertyField(prop);
                }
             }
             prop.Reset();
         }
         boxSO.ApplyModifiedProperties();
     }
     }
 }

Just replace "TestClass" with the name of your parent object's class.

Hope that helps :)