I am a granny to a baby who is nearly 5 months old. I grew up in the era of no seatbelts - when I was little, the car didn’t have any seatbelts, and then cars started to have front seatbelts but no rear ones, so mum and dad were safely strapped in while dsis and I were loose in the back. I even remember dsis stretching out along the back seat to sleep, and me having to perch on the edge.
But once rear seatbelt kits became available in the UK, dad had them fitted, and dsis and I were actually in four point harnesses - which made me car sick, but safe.
By the time my dses were born, seatbelts for every passenger were compulsory, and all three had proper car seats, properly fitted.
So I could well be the same generation as the OP’s in-laws, and have the same experience of the history of seatbelts and car seats in the UK, and there is no way I would take my precious grandchild on my knee in the back of the car, unless it was an emergency and there was NO other option. So I don’t think there is any excuse for the OP’s PIL or BIL. Unless they have been living in a cave for more than 30 years, they have no excuse for not knowing the law, or the reasons for the law.