Hello and thank you for using JustAnswer. Most states have laws regarding car seats and seat belt usage. All you can continue to say is that you are very concerned about the safety of the children because there are so many other crazy drivers on the road. If the children are not restrained and get into a bad accident, serious injury or even death could occur and the parents may be at fault for not restraining the children properly. See if you can uncover the real reason why the parents do not want the children strapped in the car seats or in seat belts. Are they just that care free? I am wondering how their parenting skills are in general. Not restraining the children in the car seats is neglectful behavior. If you do not want to attack the issue any further with them, then you can always call your local child abuse or protective services agency and seek further specific advice. You may want to file a report and then the agency will come and investigate the parents. So you do have options.
Have a great holiday!!!