Just like keeping the Santa Claus, etc. alive, it's the same thing. When it ends, it ends and no one will ever know the difference. Nor, contrary to popular beliefs, cause any issue with trust, so stop worrying, sign them and let that be your little secret forever.
I'm so glad you have kids like that.
Not all kids are like that. Some kids do, actually, take things seriously, and if they were to find out their parents lied, they could be devastated. Alas, sometimes it's hard to tell what kind of kids you have until it's all too late.
The idea of keeping a secret, in your words, forever like this would keep me from doing it. If it's something I would just confess later, then what's the point of it? If it might cause questions from an older child, it could actually work against me later on. If it becomes a funny story later on, that's awesome, but that's not the sort of thing you KNOW when kids are that little.
I'd just tell them if they ask. Sometimes things get left behind, and sometimes other people pick them up. Somewhere out there maybe there's a little kid enjoying the book. Somewhere out there maybe there's a package making its way to you because you put name and address in it, and the parents of the kids that might have picked it up are decent people. When my son's new pirate hat was taken from under a ratty sweatshirt under the rental stroller at
Disneyland, it was a moment of growth for all of us, to realize that we had to be careful. He was so little but he was so big about it. Talked about the "bad pirates" that sneak around the parks, and you have to be careful. And a year or so later when DH talked us into getting DS a build your own lightsaber that was far too big for DS, and we all sat down on a bench and got up and forgot about it, and 5 minutes later it was gone gone gone, that was another moment of "no really, we have to be careful".
It was a lot of work to get the autographs, but I assume you got more from the character meals and meetings than JUST the autographs. They had experiences, you got memories, and you got pictures. The signatures are just signatures.
To keep this secret, this silly secret, "forever", just seems pointless. It's signatures. Focus on what you did get from it, work on being more careful in the future, hope that someone who doesn't know about Lost and Found is sending you a package soon to arrive, and go make a photobook of those pictures. Those are far more important than the signatures.
