Can you give a link to these cases? All I can find is conjecture.
You might find something on
http://www.bailii.org but I didn't spot it among all the motorists arguing about liability for damaging their passenger doors in other ways
I did find an Irish case (and we still share similar laws in many things, originally from UK rule and now from membership of the EU and - relevantly for traffic law - the UN) [2013] IEHC 505 where a driver was held not to be liable for the passengers egging nearby people, but would have been "Had there been any evidence of a discussion of “egging” pedestrians." "This may well be selective amnesia but there is no evidence that the driver was warned or ought to have known that this was a likelihood. I am of the view that it would be unreasonable to import upon him the knowledge that an egg was going to be thrown or might be thrown in the direction of the plaintiff from his car." "Therefore, I with regret I must dismiss this action."
ETA: Earlier in the same: "In Curley v. Mannion [1965] I.R. 543, the defendant who was the owner and driver of a motor vehicle which was parked on its correct side of the road was held liable to a cyclist pedestrian who suddenly without warning was hit by the passenger door being opened by a passenger child of the defendant who did not look to see the coast was clear."
I would be surprised if an English driver wasn't held liable for not warning their passenger about what is visible in the driver's mirrors, but I can't find cases of it. It may be so widely accepted that no-one has disputed it for so long that there are no easily-searchable cases.