from a vet on the web

“The kitten will knead the mother whilst feeding as this stimulates the milk flow.”

Makes sense. The vet continues with more good advice:

“Some cats want to be with their human all the time. They stay in physical contact if possible and are vocal in demanding attention. Many of these cats knead and suck their owner’s clothes when they are being held. They may show signs of stress if they are left alone or do not get enough attention. Overly dependent behaviour in a cat can occur if the owner’s attitude to the cat encourages it, or if it has intensive nursing.

Start trying to teach the kitten to be more independent and when it starts to knead, discourage this. Try to distract it from what it is doing and get its attention on something else. Maybe get a tickle feather and play with this as soon as it starts to knead.

Playing games is beneficial for kittens. However, if it gets too excited or aggressive stop and walk away. If it plays gently, make sure you praise it in order to reinforce this good behaviour. You can also reward with a tiny treat.
Unwanted behaviour must be distracted – a sharp noise like some stones shaken in a tin or dropping something on the floor.”