Tell your child in a familiar setting, and do your best to keep calm. Kids pick up on your energy, and that can lead to worry and unease.
Consider specific therapy for your child to address their feelings about the divorce and to help them process those feelings in a way that is most beneficial to their needs and abilities. Understand that it is normal for a special needs child to experience an exacerbation of symptoms, but this is almost always temporary. Stay the course. The most important thing we can do is make our kids feel safe and reassure them that the divorce doesn't mean the end of the family; just a reorganization.