As an owner of both a Saab and a BMW, two companies that made both of these "items", I'm acutely aware that cars are not aircraft! Under normal circumstances!

Will the standard adult seatbelt always be properly fastened? And moreover, will it possibly induce an injury to the infant/child in the event it is to exert pressure on their abdomen? I looked into it some years ago and came to a conclusion that suited me well! So we may well not agree! But now, it's up to you!

Fortunately, it's very likely to not matter one iota!

Have a great trip!