Charlize Theron threw a superhero themed birthday party for her son.The 'Snow White And The Huntsman' actress celebrated Jackson's first birthday at the Coop, a play space for children, in Studio City, Los Angles, where Jackson's friends enjoyed Spiderman cupcakes and special appearances by Wonder Woman and Batman.A source told Us Weekly: "It was a really mellow party. Charlize is the nicest person on earth. She's gorgeous and relaxed and sweet."Charlize absolutely adores [Jackson]. They have the most wonderful relationship. [Jackson] was jet-lagged, so when everybody sang 'Happy Birthday,' they had to whisper it. She didn't want it to be really loud."It was previously revealed Charlize received nine copies of baby book 'Go the F**k to Sleep' when she adopted Jackson in March 2012.The Oscar-winning actress is famed for her love of swearing and as a joke nine of her pals each bought her the novel, which is described as a "children's book for adults".The book examines the angst involved in getting babies to go to sleep after spending hours trying to soothe them.