Charlize Theron threw a superhero themed birthday party for her son.

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.