It's working as designed. The design isn't to let your kids or other folks use your phone without permission, it's to let you give them access to sandboxed environment that keeps the other parts of your phone safe and secure. This is also why, after using Kid's Corner, the phone will prompt for the PIN again even if the phone was used within your 15-minute timeout (because the phone "knows" that it was just in the hands of an untrusted user.) The current design is what some users want, but it serves the goals of 1) protecting your data, 2) protecting your battery, and 3) pacifying your kids. If would be nice if MS let the user chose to let users into Kid's Corner with no PIN, a Kid's Corner-specific PIN, or the standard phone PIN.