The American leader smiled as he caught sight of the boy, who was the son of a military officer, whilst giving out treats.
Obama, joined by his wife Michelle, turned the media and shouted "top prize".
The first lady said to the toddler: "You were just here!", in a reference to Pope Francis' recent visit to the states.
The White House's South Lawn was turned into an enchanted forest for the event which is held every year.
Local school children were also invited to 'trick or treat' the president with the Obama family giving out cookies, fruit and baseball cards featuring their dogs Bo and Sunny.