They are finding you as their comfort and source of love an affection. It is a big compliment. they are letting you know that you are their momma and they love you enough to let down their guard and be relaxed. You probably also notice that they are giving you kitty kisses (slow blinking of the eyes when you look at them)
If they are young kitt's and you let them continue to do this, they will always kneed on you for the remainder of their life. my 12 year old boy still does this. so does the old 18 year gal. they will always tend to do this when you are occupied with something else.
its a cat law.
If you find it hurtful with their claws, a verbal Hiss or deep vocal reprimand will let them know its not ok. Or you can pick up their feet and move them off you. never ever hit them or toss them away. it will cause them fear of you and make them very insecure.