I had to look and look and look before I found it. It doesn't detract from the beauty of the quilt in any way even once you find it. Given that, and that you've already quilted it, I am adding to the chorus of leave it in!
If you don't tell them, I bet they'll never notice. It can also be fun to tell someone there is a block turned the wrong way but don't point out which one. I did that once and the recipient will sometimes look for it when he's using the quilt, but he has yet to find it. It might sound corny, but it's made the quilt more special. Tell the twins when they're old enough to play the find-the-misplaced-block game.
