What sort of poisonous substance had you in mind, that sedates a child with no noticeable immedate ill effects that can harm or kill a child in later life?
Halothane comes to mind. It causes liver problems.
"Background
Halothane and other halogenated inhalational anesthetic agents, such as enflurane, isoflurane, sevoflurane, and desflurane, are known to cause severe liver dysfunction. The National Halothane Study, a retrospective analysis, reviewed the incidence and mortality rates of postoperative hepatic necrosis from 1959-1962. [1] This study found that, of 82 cases of fatal hepatic necrosis, 9 cases were deemed likely to be drug induced. Seven of the 9 patients had received halothane. Based on this study, the risk of fatal halothane hepatotoxicity was estimated to be 1 in 35,000. When the World Health Organization (WHO) drug monitoring database was reviewed for the medications that most commonly cause fatal hepatotoxicity; halothane was one of the 10 most common causes. Given this risk, halothane is not recommended for use in adults."
https://emedicine.medscape.com/article/166232-overview