One of my kittens keep peeing on my bed help?

they are six weeks old and still with mummy cat, i confined them to my bed room which has three cat litter trays every morning i can tell mummy cat has weed and poohed in one tray and one kitten has weed and poohed in another but dont seem to find signs of pooh from the other and one keeps peeing on m bed this morning even on me iv washed my bedding as this isnt first time also dont know which kitten is doing it im planning t keep both and mummy is getting spayed this sat what should i do i know this question has prob been asked before but im getting really fed up of getting into bed to find it wet and need changing! ps i washed duvet too! thanking you for any advice lv sara.xx