This case report presents three unrelated children found to have heroin and/or fentanyl in their systems after general unknown systematic toxicological analysis (STA). The first case involves an 11-month-old male found unresponsive at their residence. The scene response suggested a potentially unsafe sleeping condition or a sudden unexplained infant death. The second case is a 14-month-old female found unresponsive after eating soft candies, suggesting that a choking related death may have occurred. The third case is a 12-year-old male found unresponsive in bed and foaming from the mouth. Gum was removed from the child's airway, suggesting another choking related death. The STA included a 14-drug category enzyme linked immunosorbant assay (ELISA) screening in whole blood. Cases 1 and 3 were presumptively positive for fentanyl, while Case 2 was presumptively positive for opiates and fentanyl. Reflex confirmation was performed in blood, urine and gastric contents, by solid-phase extraction (SPE) for 12 opiates including morphine and 6-monoacetylmorphine (6MAM) by gas chromatography-mass spectrometry (GC-MS) and for fentanyl, norfentanyl, and novel analogs, by liquid chromatography tandem mass spectrometry (LC-MS-MS). High concentrations of fentanyl and 6MAM in the gastric contents of Case 1, along with the presence of diacetylmorphine, suggested probable enteral ingestion of heroin and fentanyl, separately or in a combined formulation. Interpretation of the toxicology results could not determine a probable route of exposure to heroin/fentanyl in Case 2, however, the cause of death was clearly related to this drug mixture. In Case 3, the presence of acetylfentanyl suggested an illicit fentanyl exposure. The intention of this case report is to demonstrate the need for a STA approach for all non-trauma postmortem cases regardless of case circumstances, age or suspicion of drug use.