Hello, Julia!
My kids have been avid Disney Pin
collectors, too (can't get them to part with 'em, so trading is out!) However, we've long referred to this great set of
Questions and Answers about Pin Trading on Disney's website. Take a look...I think you'll be fascinated!
That same site explains that what you're referring to above is a called a
Hidden Mickey Pin. The number of pins in the series is included in the back stamp on the pin. Read through the article before deciding if these are keepers for your boys. As with all pins, though, the real value for the collector is finding and keeping your favorites. So, if any of your Hidden Mickey Pins include favorite characters or icons, be sure to tuck those safely away for now. :-)
Have fun planning your next trip! Be sure to send more questions, too. I'd love to help!