Making a statement about a product that would discourage people from buying it constitutes speaking loshon hara as it could harm the livelihood of either the manufacturer or the storekeeper who sells it. (In certain situations, it would be permissible).
Similarly, it is forbidden to make a cynical comment about a speaker that could either discourage people from attending his lectures, or weaken the impact of his message.
As with any mitzvah, a child should be trained to avoid speaking loshon hara as soon as he reaches the age of understanding in this regard. It is common for children to denigrate the food they are served, causing hurt to the cook. Children should be trained to avoid making such comments.
Sefer Chofetz Chaim, 22 Tishrei, page 92