Thank you Paul for your reply.
These boys are ages 12 and 13, and one is a height of about 4' 11" while the older boy is 5' 1" approximately. I believe this puts them both in the category in which I am the one required to ensure they utilize the seatbelts, and held legally liable if they don't. The car has no child seats but is equipped with the standard seat belts in the front and back seats.
In 1983 I was still living in the UK but I was not a driver in those days, so unfortunately now that I'm back, all this is new to me and I'm having to learn along the way before someone hoodwinks me into believing a lie, and bam, I get a smudge on my driving history, at the least, at worst, someone is injured should an impact occur.
Thanks again, WebyJ and Paul for the information; I appreciate the time and trouble.