You get a pair of boys wanting cool mature costumes and according to them nothings cooler about being an adult than being able to buy and drink alcohol. So you decide to give them costumes based on German beer girls, with a few extra bonuses.
Such as people will be happy to sell them alcohol and never ask for ID. Their parents will never get mad at them for drinking or getting drunk no matter how much they do it nor will they try stop them. Anytime they get drunk the girls will get extremely horny and have sex with almost anyone especially if their offered more booze in return. They'll never suffer any long term effects of excessive drinking or alcoholism like getting fat or having liver problems. And the last one which you make sure they memorize before they leave, they'll always get a discount on any boozes as long as they buy them well wearing their costumes insuring they were the costume as often as possible.
After the transformation the new women pay and run off saying they're going to the liquor store to buy some beer.