It is one of the saddest tragedy that children regret in the later days not being able to regurgitate parents' love before they pass away.
最大的悲剧,这里我用了"one of the saddest tragedy",指最悲伤的悲剧之一,也可以直接换成"the saddest tragedy".
子欲养而亲不待,意为”孩子后悔无法在父母过世之前反哺他们所给予的",是而有了以上的翻译.
如果喜欢直译成”孩子想反哺父母,但父母已不在”,也可以用这样一句（更简单直白一些）
It is the saddest tragedy that the son wants to serve his parents in their old age,but they are no more.
两句都可以,就看偏向什么风格了.个人偏向第一句,因为句式更漂亮,句意更恰切.