Service animals (usually dogs) are one of the accomodations provided under the Americans with Disabilities Act of 1990. A disabled person's physician must give a written prescription that states the person should be allowed to have the animal with him at all times. A service dog may assist people in wheelchairs, deaf or blind persons, disabled people who wonder away or assist a disabled person to stay calm. There are many uses for a service dog.
An internet search will reveal much information on the topic of service dogs. Two sites you might start with are:
Armed with information about service dogs, my advice is think before you do it. A service dog benefits Billy Ray greatly. I think it is an important part of his care. However, maintaining a dog is extra stress and work. Part of the reason it works for Billy Ray is because I love animals so it is a labor of love. Not everyone will feel that way. You need to be prepared for that both physically and financially. Sometimes you can obtain financial assistance and sometimes not. You also need to explore the benefits to your child in terms of his/her reactions to a dog. Would it help significantly enough to make the added stress worth it? In our case it does but that may not be the same for your child.
My advice is that you be sure before you traumatize your child and a dog that you are ready to deal with it.