How do I add a dependent for blindness

Dependents are added in MY INFO.   However, there is no extra credit or deduction for a dependent who is blind.   If the taxpayer or the taxpayer's spouse is legally blind, there is an extra amount added to their standard deduction.  Nothing is added for a dependent's vision impairment.

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.