When we got our boys, the rehoming woman advised us to use the wood pellet stuff. My only gripe with it was it turning to sawdust when wet. Neither of my boys tried to eat it, they did flick it out of the litter tray and play with it. If yours are three weeks old, presumably they'll still be with mum? She should stop them doing that I would hope.
I've never heard of a cat eating litter before, but I do seem to remember people using shredded newspaper for tiny kittens before, maybe that's why? We use Catsan, which is sort of pebble/grit type stuff and consequently looks very unappetising. Also, I don't think it's cat behaviour to eat where they shit, so hopefully that should put them off?
hmm,i've never heard that before, we often have kittens, as we rescue them from a local farm and re-home them, we use the cheap tesco stuff,and have found it to be fine. they all start off eating (well chewing) the litter when they are little - they are just exploring it,then they start eating the food and using the tray properly lol!