I have 2 young boys that tend to lose pins :\
We first tried the ones they sell at the park, but I wanted it on ALL the pins, and it's really difficult to use the tiny key to unlock them all the time. I found perfect ones on ebay, the seller is disneypatriot. I've bought them from him 3 times, as well as numerous friends. They are easy enough that my 5 and 9 yr old can take them on and off.
Good luck with whichever you choose!
Leana