Totally get that. I told my kids after they knew it was their job to help keep the magic for the kids who still don't. I just used the comment that "you wouldn't want someone to have ruined your magic so don't be that kid and ruin someone else's" etc.

Funny part is my step daughter knew before my daughter, and my step daughter was the one who was a bit bitter and hurt and so we used exactly this to make sure she didn't hurt my daughters feelings by spoiling it. She got into it after a little while and was having a blast helping us do creative things with the elf on the shelf that year before my daughter figured it all out.

All good times, kids are awesome.