My iPad mini pass code wont open

I bought a iPad mini for my kids last week. We downloaded a few games and it was great. Now we can't log on. It has a pass code. It seems like the touch sensor may be off a little. It takes a few tries and I kind of have to press and hold to get the button to slide to unlock, then no numbers will show up when we try the pass code. The number will outline itself so I know I'm putting in the right code, but it won't open. Any suggestions before going back to the store.......thanks so much