I believe grocery stores have the right to arrange their stores in whichever way they choose, within the law. The grocery stores set up their stores in a way they think will be most profitable and if they think alcohol belongs next to the Halloween aisle it is their decision. I agree, morally it is wrong to have beer next to kids' costumes, however they probably get a lot of business from those having Halloween parties and buying alcohol.
Every kid in America has been in contact with alcohol once in his or her life, either from being at a state fair, or seeing it in a commercial advertising a beverage. There is no reason to prohibit a company from setting up their store in a profitable manner.