Your child qualifies as a dependent if the doctor certified as such... Per IRS: "You are permanently and totally disabled if you cannot engage in any substantial gainful activity because of your physical or mental condition. A qualified physician must certify that the condition has lasted or can be expected to last continuously for 12 months or more, or that the condition can be expected to result in death".
Please note, however, that "being disabled" doesn't provide you with any additional tax credit/deduction for tax purposes at this point. A special tax break for disabled children is the fact that they qualify as dependents, and qualify for other tax benefits, such as child tax credit, earned income credit no matter how old they are as long as they live at home with their parents. But it doesn't give you any more money or deductions.
