Kittens peeing on bed

Question
We have 2 kittens, both male, we've had them for 5 weeks now.  They have been using the litter tray from the moment we got them and have never peed or pooped anywhere other, but this weekend, they peed on our bed 1st thing in the morning... the duvet was taken to the dry cleaner and a new one put on, but then this morning one of them did it again!  (We're not sure which one)

During the day if we are not in they have free run of the kitchen, hall and spare room, at night they are left in the kitchen.

Is it excitement when they are let into the bedroom, are they marking their territory or just being naughty?!!!  They've been as good as gold until now!

Please help!!

Answer
Hi Zeenat,

They are still young. They need another litter box in the area. It sounds like they are playing so hard that they forget to go. Like little children the "bathroom" is too far away. I believe that this will take care of the problem.
Oh, you need to clean the area where the one peed with an enzymatic urine cleaner. This removes all smells from the kitty urine so they won't be tempted to use that area again. You can get this at any store. Good luck.

Ciao, Karen