I discovered the solution to carrying children by accident "back in the day". I found if you cram three of them in the back seat it stops them rolling about on the bends.
Seriously though, back in the old days I didn't think about the safety implications.
The rules for a car without seatbelts are that they aren't allowed to be in it under the age of three and older than that they must be carried in the back. With the front seatbelts of a UK 500 I take it that normal rules about the age/height of children would apply.
I think it is unlikely even that the front seatbelts would be much help in a crash, especially if the original type seats are fitted which would simultaneously try to eject you through the windscreen.
The back seat is so weakly fitted to the car, with a thinner rubber strap for the backrest and two slots for the squab, this would also fight to push you forward, even if well secured belts were fitted.
I
do take my small grandchildren in the car for a treat, mainly on the driveway where even the three year old has "driven" it a few metres standing on the seat and ejecting all the washer fluid whilst annoying the neighbourhood with the horn.
I have occasionally dropped them off at school and driven them to the beach (one and a half miles) a few times.
I think you just have to be very cautious with those precious objects inside your precious object. But I doubt that fitting seatbelts makes for much increased safety in a car designed in the fifties and made from a catering size, empty baked beans can.