Please help me understand why Noah cursed an unborn child.

Noah cursed Cainan, because it was Cainan that did wrong. Let's look at the verse again. Gen 9:20  And Noah began to be an husbandman, and he planted a vineyard: 
Gen 9:21  And he drank of the wine, and was drunken; and ((he [was] uncovered within his tent.)) ....... No mention of who did the uncovering, yet......Gen 9:22  And Ham, the father of Canaan, ((saw the nakedness of his father,)) .... cannot really phrase that any other way, He did not uncover his father's nakedness, he saw it.)........and told his two brethren without. ........Ham saw, His father naked in his tent, and , told his brothers. Now if You had committed such a heinous act as you accuse Ham of doing, would you just go and tell your brothers about it. ? well would you? everyone wants to blame Ham because from him came the black nations. But continue reading the evidence..... Gen 9:23  And Shem and Japheth took a garment, and laid it upon both their shoulders, and went backward, and covered the nakedness of their father; and their faces were backward, and they saw not their father's nakedness. ... So Ham tells His brothers ( and this is more likley what happened..) "Hey guys, just saw Cainan coming out of Dad's tent, so I pop in to see the old man, but He is in a drunken sleep and naked. can you guys get him covered I am going to have a word with Cainain." ......Gen 9:24  And Noah awoke from his wine, and knew what his younger son had done unto him. Please note, Noah's wife was not in the tent needing to be covered.
Ham was the oldest of the three boys, so Noah would not have referred to him as his younger son. This was a term used for grandchildren still deemed as their offspring ( Ref: Jacob and his crook uncle. when Jacob decided to leave him ) Ham told his father what had happened to him. Noah's curse was on Canaan alone, And the curse would not have followed the generations. even God, only goes to the forth generation of them that hate Him. Exo 20:5......... visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;  ...... Cainan was already born and grown, The bible just does not say how long it took Noah to plant and reap a harvest from the vineyard, and the time it took for the grape juice to turn into fine potent wine.

nope!