That's life with kids. Everything we have disappears. And it turns up in the strangest places. Sometimes because we put it there - while our brain is in "gotta pick up the kid from soccer practice, make dinner, check homework . . " mode. Then there's the kid who steals everything . . . and hordes it.
I can't help you with everything, but I've found a funky (you'll see) answer for the Case of the Disappearing Keys. I'm no Cam Jansen, but I found the Fun-Key Finder courtesy of its Hawaii-based inventor. She signs her e-mails Aloha. I love that. Why can't New York have its own Hello and Goodbye? Oh yeah, we have "F*** you D***head." Gotta love New York!
Ok, sorry. Where was I? Mommy brain strikes again. The Fun-Key Finder loops around your purse handle (or stroller handle or pretty much anything you lug around). At the end of the leather loops is a clasp that opens easily (no struggling with another blessed key ring that you can't get your stubby Mom nails under to get those fat rubber car key ends on). You clip your keys on when you go in the store, and when you come out, no rooting around in the purse - or worse, wondering if you left them on the banana rack.
So chuck the pipe cleaner lanyard they made at pre-school that looks like it's been through the wars and still ends up getting lost. The Fun-Key Finder is the find of the week.