There are no alcoholic beverage allowed anywhere in the Magic Kingdom.
Beer, wine, and spirits are readily available elsewhere at Walt Disney World. These items are sold in the gift shops of every Walt Disney World hotel, as well as at many of the restaurants at Epcot, Disney's Hollywood Studios, Animal Kingdom, and at the water parks and Downtown Disney.
There are kiosks or stands at these areas that sell beer. You may walk around the parks (except the Magic Kingdom) with an adult beverage in hand. You do not need to be seated in a restaurant to enjoy your drink. Epcot is a particularly festive place for beer lovers. You can read about this in the post found
HERE.
You should know that while you can buy beer at three of the four theme parks, you cannot carry it into the parks yourself. Guests may bring food and non-alcoholic beverages into the parks with them, but the transport of alcohol into the parks is prohibited.