Seeing as a "betting shop" or a "retail betting site" is akin to a "brick&mortar" shop in terms of the way the bets are being placed as opposed to "online betting", I'd say the right translation to "retail gambling" is "apuestas en casa de apuestas". Maybe picking a suitable synonym for "casa de apuestas" so that it doesn't sound kinda redundant would be nice. None comes to mind, though. "Pequeños apostadores" sounds... coherent, but I wouldn't bet my life on it without a strong consensus being reached. And shown to me, of course.