what nobody's said is that if the child (or anyone else) has whiplash, they need to get to the doctor straight away, as no insurance company in the world (surely) will pay out on a whiplash claim without sound medical evidence backing up that the person HAS got whiplash, and that it arose as a result of the accident. If the child had no seat belt, that's a reason why the insurers are very likely not to pay out, or to reduce the claim if it were successful. Were the police involved? it would seem to be sensible if the old couple notified them, this sounds like it could get messy - not only was she backing onto a main road without due care, but her child wasn't belted up.
Btw I have lived in houses where I have had to back out onto a main road - backing in would have been even more complicated - you do it very careful!