To be perfectly honest, this is why I don't teach DD any math outside of school. After I taught her subtraction with carrying in about 10 minutes because she asked (I forget how old she was, but not old enough to be covering this), it hit me that I should probably shut up.
Sometimes it's better to just leave it alone. If you can't get in-class enrichment at their level, you risk creating too much of a discrepancy between what they can do Vs what's being taught and it causes all kinds of attention issues.
Sometimes you can't leave it alone. DS has brought home unfinished worksheets from class that he was too busy staring out the window to do. Getting him to do them at home was torture. One day he said "It's so boring! Can you make it harder?" "...um, well no, but... if you finish this sheet that your teacher wants then I can give you a different sheet that's harder." Presto: motivated kid, and completed worksheet.
