The 3 Suspects

Riddle: one day a young lady walked into her living room and saw her son dead. she called the police and told them that she was most likely sleeping when he died.the maid the butler and the father were the only suspects.the made was a nice young women who loved kids,the butler was a greety old man and the father was a old man who didn't have a leg and he didn't have a proctetic one either.the maid said she was outside in the garden,the butler said he was outside cleaning the pool and the father said he was practicing for a running marathon. who could have killed the boy.