The children had not smoked the synthetic drugs, and while inquiries are continuing, at this stage it appears they ingested the drugs "through the food that they ate for dinner".
No drugs were found at the house, Ferguson said.
"Police are still trying to unravel the facts. In part that is being made difficult by, we believe family members hold the key to this, and that someone from within that family group knows what has happened but to date that information hasn't [come out]."
The children were released from hospital five or six days after being admitted, but are now being cared for by another family member.
"The medical people were extremely concerned, particularly by the middle child's condition. They held the greatest concern and everyone is thankful that he was able to pull through."
The message is not getting through. This is fatal stuff.
Ferguson said the child was a "very lucky boy".
Synthetic cannabis was first suspected by the paediatrician in charge, who recognised the symptoms the children were displaying. Urgent testing and analysis proved the suspicion to be correct.
Police and the chief coroner have revealed that 20 people have died so far this year as a result of suspected synthetic drug use.
Ferguson said this case highlighted the "absolute risk" people were taking by using synthetic drugs.
"I just wish people would listen to the warnings being given, because we've had too many deaths lately.
"The message is not getting through. This is fatal stuff.
"People need to leave this stuff alone. It's killing people up and down the country."