1. First and foremost, the income a single child can earn in a situation like this I refuse to believe could take the family from "desperate" to "okay." This won't raise the family from poverty; in fact, it sustains poverty, because a child who is working and not learning to read, or respect himself, will stay in poverty. It just makes an intolerable thing slightly more tolerable because the act of trying to change the situation is there.
2. The grape boycott Caesar Chavez influenced resulted in California having the best labor-friendly agricultural laws in the country (although still pathetically inadequate). Cocoa beans are a ~$7.1 billion* dollar/year industry; processed chocolate is much larger than that. So while, yes, the farms could switch to a different crop, or the farms could go bankrupt, there are very concerned interests ensuring that doesn't happen. But they will only be willing to come to the negotiating table if pressure is put on their wallets.
*(very rough estimate based on Wikipedia's production numbers and the average value of Napa County grapes at $2000/ton)