how to unlock my sim.

Hello, I want to unlock my sim card since the phone is already lock. If turned off my phone or battery is empty, it is off and if i put it on again, I need to type my 4 digits code to unlock the sim. last time the kids tried many times so KPN is asking my PUK code. I was able to unlock it again, BUT now my 4 digits code doesn't work anymore. It has to be the PUK code it's annoying it's too long numbers. I don't need to lock my sim. HOW CAN I UNLOCK IT. thanks for reply.