I don't think a Shirley Temple is a good equivalent of a pint of mild. A Shirley Temple is a non-alcoholic drink that looks like a cocktail but isn't. It's something you order for your kid when all the adults at the table are having cocktails and you don't want the kid to feel left out. Bars have various formulas for making them; usually it's a splash of grenadine in a glass of 7-Up. I don't think it's done so much nowadays. Why would you want to encourage a kid to pretend he's drinking alcohol?

Mild, on the other hand, contains alcohol, but it's sweeter than other kinds of beer. Mild seems to have disappeared from British pubs in recent years. Maybe it lost out in the competition with lager. As for why it's mentioned in the song, my best guess is that it's meant to convey an old-fashioned taste. I'd suggest "shot of bourbon" as a better equivalent, and it seems to fit the meter.