My DS is 2.8 and I have noticed that he is very knock kneed. I asked him to stand with his legs together (naked) and he put his feet together but one knee was in front of the other - very contorted. So then I stood him with his knees together and his feet were a good two inches apart. His feet also lean in somewhat although when he wears shoes this is not really noticeable.
DS is quite big boned and has big knees. I found that I had only about a half inch gap between my knees (less than the norm for a 2-4 yr old according to the NHS birth to five book) and DH then claimed that he was knock kneed. So big hereditary factor. But I wondered if I should mention it to doctor or health visitor because DS seems quite bad.