That's done on purpose. The ability to kill off villagers on a whim is not considered desirable in a game intended for all ages. However many toxic potions he is given, they will not cause him to die, even if his health bar appears completely empty. A villager in such a position, though, would be highly vulnerable if he fell sick or injured himself working.

Luckily, children can't work, so cannot injure themselves, and so that route to infanticide is not an option.

Give the poor kid a chance! Why do you describe him as 'lazy'? Lots of my villagers have grown up to be fine members of society from unpromising beginnings.