1. He or she cannot engage in any substantial gainful activity because of a physical or mental condition.
2. A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.
You should have a statement from a doctor describing the above.
So sorry to hear about your child's health problems. There is not any extra tax benefit or deduction to saying that a child under age 13 is disabled on your Federal return. If you have a disabled child over 12 and need to pay for childcare so you can work, saying the child is disabled can enable you to claim the childcare credit. Or if the child is 19 or older, you can still claim the disabled child as your dependent. Otherwise, it does not benefit you to claim that your child is disabled. Your state tax laws may be different.
Of course, any medical expenses you are paying out of pocket should be entered in Medical Expenses in case you have met the threshold for claiming medical expenses as a deduction.