Cats have an innate tendency to munch on leaves to cure all sorts of ailments (furball, fiber for their digestive system, vitamin supplement, etc).
The best way to prevent them from munching on the orchids is to provide them with their own supply of vegetation that they will prefer over the orchids.
Pet shops usually sell potted herbs or grasses that cats love to nibble on.
If they have these in abundant supply, they will most likely stick with those as opposed to munching on orchids that have relatively tougher leaves ( and I hear the leaves taste yucky too )
You will probably just need to stick to your guns and force the kitten to get litter-trained and always use the litter box and discourage them from using the bark as kitty litter.