The assumption of that day was this man had been cursed by God with blindness because of some sin committed by his parents or for some sin the man himself committed while he was still in the womb. God is not in the cursing business, but in blessing. The verse in question occurs within a passage dealing with idolatry and immorality. This passage has nothing to do with why a child is born challenged. The problem is this line of thinking is simply bad theology.
Life is tough.
God never promised us anything different than that. Sometimes difficult, even horrendous things, just happen. God did not send the drunk driver out to kill the newly wed. Rather, someone was simply irresponsible, even evil, and they got into a car under the influence. Similarly, God did not send us challenged children.