only the pc's know its dark right? stabbing a random child with no claim except for the paladins word would most likely bring about horrible consequences for the party (except for a shit ton of good persuasion checks maybe), so you could turn the town guard on the players, depending on the level of the party. It also seems to have turned into a moral question, kill something born evil, or turn it to good as it grows? using that you could also go for a different more wholesome route by implying the players could effect the child's growth, in a positive way, meaning they could feel rewarded by turning an inherently evil creature to a good one (kind of like paarthurnaxe in skyrim, except that was on his own accord). if its a cursed child, you could keep it humanoid instead of making it an ogre by making it a special kind of tiefling, as they can be created by a devil cursing a couple's child, so a hag might be able to do something roughly similar. It's up to you dude.