Knock knee is noticeable when a child stands up straight with their knees together. Their lower legs will spread out so that the ankles are much further apart than normal. In children between the ages of two and four a gap of up to 6cm is considered normal, in older children the ankles can be up to 10cm apart without cause for concern. A small degree of 'knocking' of the knees is normal, however it is worth seeing your child's GP for advice if they show any of the following signs: