The way it is designed by Apple the iPhone will only pair with another device via Bluetooth if it has a supported reason to pair. The supported reasons are shown in the link provided by Menel. I believe the only supported reason for iPhone to iPad pairing is tethering (i.e., sharing your iPhone cellular internet connection with your iPad).
If you don't answer the question that has been asked several times it will be hard to help you: Why are you trying to connect your iPhone to your iPad via Bluetooth? i.e. what do you intend to do over the bluetooth link once the two devices are paired?
If, for example, you are trying to pair the two devices in order to copy data (pictures or other files) from one to the other via Bluetooth, it will not do that as a standard feature.