5wk kittens sleeping in litter box?!

My cats second litter (speying booked for as soon as kittens old enough), 6 kittens after just 3 last time. All thriving, climbing out of their box for almost a week & seen sampling the food last couple if days, so have been putting them on litter tray when they wake up. Have been finding tiny puddles in there so at least one or two of them getting the message, but today I went in and 2 of the curled up asleep in the tray?! I took them out and put them in with mum, anything else I should be doing? Thanks

Kittens loves to sleep and play in the litter box? What litter are you using? Is it clumping?

It's 'worlds best litter' tried a lot of different kinds but this has worked best for us so far. Would clumping litter make them more likely to want to sleep in it?!

I asked because sometimes kittens like to eat the litter and causes a serious problem, i recommend non-clumping for kittens because of it

Worlds best is clumping litter made from natural corn kernels so harmless if ingested (looked into it before the kittens came along) don't know if it's available in the US (I'm in the uk). I'm going to try to spend as much time as possible in the room with them over next few days during their waking time to try and get them using the tray, it hard to keep track when there are so many! They all seem happy to sit/stand/sleep in it though, as it has been used even after clumps removed some small traces remain which will hopefully help their little noses associate it with the right activity! Early days I guess