IRS definition for a permanently and totally disabled child - https://www.irs.gov/pub/irs-pdf/p501.pdf#page=13
Permanently and totally disabled.
Your child is permanently and totally disabled if both of the following apply.
• Your child can't engage in any substantial gainful activity because of a physical or mental condition.
• A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
If the child is under the age of 19 indicating they are disabled has no tax benefit. Unless the child is age 13 or older and you have dependent care expenses so that you can work.
Do I get a credit or deduction if my child is disabled?
If your child is older than 12 and you need to pay someone to take care of him while you work, you can say he is disabled and you can still get the childcare credit on your federal return. Or if he is 19 or older you may still be able to claim him as a qualifying child dependent on your federal return.
Aside from those things, there is no federal tax advantage to saying that your child is disabled. Your state tax laws might be different.