At what age can my daughter say she does not want to go to her Dad's house for visits? We are in Kansas. I am custodial parent and he has every other weekend and one night per week. She does not want to go anymore.

18.

The only way to change visitation and custody is thought he court, you as the custodial parent is held legally responsible to have the child available for visits. ( and can be found in contempt of the court order if you don't)

So you go back to court and have the child testify and ask for a change in visitation. Now many states can and will require visitation so it may not be of any help.