This advice-column-style blog for SLPs was authored by Pam Marshalla from 2006 to 2015, the archives of which can be explored here. Use the extensive keywords list found in the right-hand column (on mobile: at the bottom of the page) to browse specific topics, or use the search feature to locate specific words or phrases throughout the entire blog.
Oral habits are difficult to change when cognition is low.
The standard rule of thumb is to recognize that he child is benefitting in some way from the habit. Most people postulate that the grinding relieves boredom and/or it gives tactile stimulation that feels good. Many children with low cognition also have low oral-tactile awareness and discrimination, therefore oral habits feel good.
If grinding is relieving her boredom, then she needs to be in more stimulating situations. Perhaps she is spending too much time in a regular classroom where she in no way can engage.
If it is tactile stimulation she is seeking, then other pleasing oral-tactile stimulation and chewing activities may help curb the chewing. Use powerful input like cold and vibration.