Yes- It is the Law.
What if they were kids- would that have been any different?
If you were in a accident and they were hurt, you as the driver of the car would be held responsable.
If a Cop saw you, as the driver, YOU would get the ticket. ("click it or ticket" is the slogan here in Cali)
Also- unbelted people can also be "flying" people, and harm others in the way if you are in a crash- and it is the unbelted people that almost always die- even when others walk away without a scratch.
My Grandparents never wanted to buckle up when they got in the car with us, and my parents refused to start the car and go until everyone was buckled up. They were "put out" at first, but my parents were driving, and it was "my car my rules" and that is the way it is.
It is harder for older people to get this sometimes. They grew up in a era that didn't have seatbelts most of the time- the cars didn't even COME with seatbelts- let alone have a law about using them. They are required, and if you are driving them, tell them that as a responceable driver you will not drive when you are drunk, when you are tired, or when others in the car are not buckled up.
It is better to make them mad, than to have them die (or you get a huge fine)