Adds a LOD Group to everything you select and sets the children correctly.

LODTools.cs

using UnityEditor;
using UnityEngine;

public class LODTools
{
    [MenuItem("Tools/Add LOD Group/LOD 3")]
    private static void AddGroup()
    {
        var objects = Selection.objects;
        foreach (var ob in objects)
        {
            var go = (GameObject)ob;
            var group = go.AddComponent<LODGroup>();
            var lods = new LOD[go.transform.childCount];
            var transitions = new float[] { .6f, .3f, .1f, .01f };
            for (int i = 0; i < lods.Length; i++)
            {
                lods[i] = new LOD(transitions[i], go.transform.GetChild(i).GetComponents<Renderer>());
            }
            group.SetLODs(lods);
            Undo.AddComponent<LODGroup>(go);
        }
    }
}