Does a child diagnosed with Autism qualify as disabled on your federal tax returns?

Disability for a child doesn’t affect your return since the definition refers to the ability to work. 

If the child is under the age of 19 checking the disabled box will have no effect on your tax return. 

IRS definition of disabled -

A person is permanently and totally disabled if, at any time in 2021, the person can't engage in any substantial gainful activity because of a physical or mental condition and a doctor has determined that this condition

has lasted or can be expected to last continuously for at least a year or can be expected to lead to death.