It's not a problem at NOW TV's end its because the Xbox parental controls that are locking out the content above a certain certificate level. You would have set this level yourself for your children when setting up the accounts. If both you (adult user) and a child user are logged in at the same time the Xbox enforces the child setting as it doesn't know who's trying to use the NOW TV app. Make sure the child user is signed out and only you as an adult user are logged in when you use the app and you should be fine.
