It looked to me more like WOODY Nightshade, except that woody nightshade usually has mauve flowers. If potatoes 'go wild' and produce fruit, it is very similar (with white flowers), and also toxic. Woody nightshade and 'potato apples' are somewhat toxic, but not in the same league as belladonna. Get rid of it anyway, but don't panic. It would take a number of berries (and I believe they are bitter) to do worse than cause a nasty tummy upset. I would suggest TELLING him that this one is defintely not fit to eat, and will make him feel ill, and then get him to help find all of them to treat with a 'touch' weedkiller. You'd be surprised how well children can manage that sort of thing.

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Flowers come in too many colours to see the world in black-and-white.