It is quite common for kitties to sample kitty litter. They also like to lie in the litter pan and when they finally get out, they lick themselves clean. If you are using clumping litter, it would be wise to discontinue and use another type, something biodegradeable that won't hurt them if ingested. Clumping litter has been shown to cause problems in young kittens as well as cats. It can cause the kittens to vomit, lose weight and cause internal blockages as well ( among other unpleasant things)
Your kittens all need to see a vet, and yes, I understand that money is an issue for you and money is tight. But when you open your home to an animal, you take on the responsibility of caring for that small being, the same as you would a child. You should take this family to the vet for a workup and check-up and let the professional that can see the problem help you.
I do disagree with Dragon Lady's advice about removing litter material. The kittens need to observe mom using the box that is how they learn they have to. Either switch to a friendlier brand of litter, or just strip out pieces of newspaper and use that.