SJS – it’s absolutely true that calling someone in a respectful manner while acting towards them with chutzoah and impudence is rather hollow.
However, allowing a kid to call by first name will diminish his overall respect for adults in general as he sees himself on equal footing, on a first name basis. This affects the aura that adults have in the eyes f children that engenders such repect. This is not to say there are no exeptions – of course there are. But from a chinuch perspective, it teaches kids a certain amount of respect for adults, something that has a basis in halacha.