If the child is under the age of 19, there is no tax benefit for indicating the child is disabled.
Here is the IRS definition for being permanently and totally disabled -
Permanently and totally disabled. Your child is permanently and totally disabled if both of the following apply.
• He or she 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.
