when they turn 18.
this is not a sarcastic answer. until the child reaches the age of 18, the parents are responsible for them. until that time, it is the responsibility of the CP to ensure the relationship with the child and the NCP. it is your biggest job.
realistically though, you can force the child to go but in the end it only makes the child resent the NCP for making them be there.
as the child gets older. things are going to come up and happen that the child will not be interested in visiting with the NCP. friends, parties, going out with the gang, etc. it is a hard fact to handle. but unless the NCP is willing to let the visitations go, then you should work with the NCP to ensure that the child goes.