ソースコード

using System.Collections.Generic;
using UnityEngine;

public static class GameObjectExtensions
{
    /// <summary>
    /// 孫オブジェクトを除くすべての子オブジェクトを返します
    /// </summary>
    public static GameObject[] GetChildrenWithoutGrandchild( 
        this GameObject self 
    )
    {
        var result = new List<GameObject>();
        foreach ( Transform n in self.transform )
        {
            result.Add( n.gameObject );
        }
        return result.ToArray();
    }
}

使い方

var children = gameObject.GetChildrenWithoutGrandchild();

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