For income tax purposes, disabled means "unable to perform substantial gainful work."
A person is permanently and totally disabled if both 1 and 2 below apply.
1. He or she cannot engage in any substantial gainful activity because of a physical or mental condition and
2. A physician determines that the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
Many people who have medically recognized disabilities can still perform gainful work and are not disabled for tax purposes. Being able to work a minimum wage job without significant accommodations is considered proof of being able to perform gainful work, although working in a sheltered workshop for people with disabilities doesn't count. You and your child's doctor will have to determine if, based on her specific situation, she is able or unable to perform gainful work.
For a dependent, being disabled will allow you to claim their childcare expenses past the age of 13, if the disability means you have to pay for childcare so you can work. And it allows you to claim the person as a "qualifying child dependent" past the age of 19. But under the age of 13, checking the disabled box provides no actual tax benefit.
https://www.irs.gov/pub/irs-pdf/p3966.pdf
