First I have to say that every state, in fact every social security office is different. What will be required will depend on the decision of the reviewers.
But I can shed light on a few things. First, diagnosis in the case you describe is irrelevant. The reason a child qualifies for SSI is because of his/her inability to function "normally". The child must be "severely" disabled in order to get SSI. In that set of papers they asked you to return was probably a questionnaire about what your child can and cannot do. They are looking for that questionnaire to indicate that your child is "severly" disabled as compared to other children his age.
Another point to remember about this first interview is that you have to provide info about your family income and assets. So your W-2s, prior tax returns, documentation of any assets including car registration will be looked at. In addition, you need to provide your and your husband's ss cards and proof of identity. I believe the same is required of ALL of your children and anyone else who lives with you. This is all to prove that you qualify under the income and assets tests.
If you want to get technical in terms of what you need to prove here's the law on ADHD and you can look up the other problems you describe on the second link which is the main page for definitions of specific disabilities in children.
112.11 Attention Deficit Hyperactivity Disorder: Manifested by developmentally inappropriate degrees of inattention, impulsiveness, and hyperactivity.
The required level of severity for these disorders is met when the requirements in both A and B are satisfied.
A. Medically documented findings of all three of the following:
1. Marked inattention; and
2. Marked impulsiveness; and
3. Marked hyperactivity;
http://www.ssa.gov/disability/professionals/bluebook/112.00-MentalDisorders-Childhood.htm#112.09 Psychoactive Substance Dependence Disorders
http://www.ssa.gov/disability/professionals/bluebook/ChildhoodListings.htm
The question then is what is "marked". This is where your paperwork becomes important. It's not just doctor's evaluations that help here. eg the SSA may want your child evalutated by their doctors. They also are typically looking for your child to have been hospitalized repeatedly, to be in a self-contained class or alternative school for his issues, to have repeated reprimands from school, etc. So providing the school placement papers such as the IEP is helpful. If you're saying that your child is mainsreamed and doesn't even have an IEP you're going to have a tough time getting SSI on the basis of ADHD. I can tell you that mine was in self-contained Special Education class from pre-school through 5th and still didn't get it on the basis of ADHD.
Bring as many medical reports as you have and names, addresses and phone numbers of any other professional who has seen him. They will send a form to all those people to get more info.
Be prepared to spend hours there. Sometimes it's quick and sometimes many hours.
