How can i stop my kittens weeing on the bed?!?

I have 2 kittens and they both wee on my bed covers not all the time though it seems to just be after they have been playing they always use the 2 litter trays but still wee on my covers and ive ended up buying 3 new quilts upto now! and ive tryed cleaning them and washing my sheets in a little citronella, ive been rewading them when they do wee in the litter box and i clean it out regulary i cant seem to find any solution to stop this, any help would be appreciated thanks.