It's a very specialized area of the law and as Sue suggested you need an attorney whose practice is estate planning and even then not all of them are going to really know what they're doing. Asking how many they have set up and how many active trusts the attorney is advising may help you find a well qualified attorney.
We have been trustee's for our difficult child 1's court-supervised special needs trust for 10 years. This is a different kind of SNT than you would set up yourselves. It is irrevocable and the funds came from the settlement of an auto accident when he was a minor.
However, the function of the trust is basically the same as one you would set up for your disabled child with your own money.
The purpose is to protect a certain amount of money for things that your disabled child may need in the future that would not be provided by state/federal programs like SSI, Medicaid, IHSS. Usually, for a medical service to be paid by the trust there must be evidence that the service has been denied for coverage by the appropriate agency. The trust can be set up at any time and is usually revocable up to the time of your death. After that, it is usually an irrevocable trust - no changes can be made in the way it is structured although trustees may be changed in the way that the trust document specifies.
Things it can be used for include educational expenses like tuition and books, computers, home modifications required to meet the adult child's special needs, attorney or other professional fees associated with management of the trust, transportation including things like taxi fare or air fare, purchase and modifications to a vehicle that is specially modified to meet the disabled person's special needs, vacations, in-home care not covered by IHSS, small thank you gifts to caregivers, art supplies - anything pretty much that isn't food, clothing, shelter or medical care. You cannot pay most utilities but you can pay for telephone costs including cell phones. The costs for those things are expected to be covered by publicly funded programs like SSI or SDI, etc. The trust also specifies what will be done with the trust proceeds in the event of the adult child's death. However, if the adult child received Medicaid or Medicare the remaining trust funds must be turned over to pay the government back for the costs to provide your adult child's medical care. At least that's the way the law stands right now. It is not evenly enforced however.
It is a good idea to create a SNT if you know or have reason to believe you may have a lump sum to leave to an adult child who you expect to be on SSI. If there is a lump sum inheritance or there is valuable property (except for a residence that the adult child will live in) or stocks inherited those will have to be liquidated and used up before SSI and other programs will kick in.
It's actually a good idea for anyone with a special needs child to put in place because none of us can know the circumstances that will arise at the time of our death. Even if you don't have a big life insurance policy or lots of money in your 401 K, if you die in an accident that results in a substantial settlement to our estate then the adult child on SSI is likely to have a problem, particularly if you die without any will at all as the inheritance laws of the state will dictate who gets how much of your estate.
It also gives you a lot of control over who is in charge of the money as the initial trustees and as successor trustees.
Patricia