Make it into a game! Tell your child to touch a part of their body while you are changing their diapers. This keeps their focus on you and the game rather than on the squishy stuff in their diapers.
As you change their diaper, continually give them these commands and they will be so excited about playing this game with you that they will forget to explore down below. This usually only holds their attention for about 1 minute, so move quickly!