If the child's seizure disorder can be documented as a disability by the neurologist like you stated, and the child's condition meets the following IRS definition of disabled, you would be able to claim your child as a dependent:
"Permanently and totally disabled means: he or she cannot 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."
Good luck with everything!