Hi,
can I ask what litter you are using? It may be that the kittens don't like the litter. Another thing you can do is use non bio washing powder where they have 'been' & then use vinegar or surgical spirit as this will stop them going there.
Do you normally feed them the same time each day? This may help them to get into a routine plus you could maybe watch them & put them in the tray after eating.
Other than that not sure what else to suggest really hope this may help!
