|For more about the Curse of Knowledge, see Made to Stick by Chip and Dan Heath|
Once you know, you cannot unknow. You cannot put yourself in the place of someone who doesn't have the knowledge that you have gained.
We build our knowledge and understanding upon what we've experienced plus insights (from those experiences). If kids haven't had experiences or built previous knowledge, they cannot understand. And they don't grasp understanding just because we've "explained" it.