"Ah, Sherlock, we were waiting for you. Something horrible has happened and we need your help."
Sherlock Holmes had barely entered the Security Council room. Only a few hours ago, he was summoned from 221B Baker Street. The mission was so urgent that Sherlock was flown across the Atlantic in a suborned SR-71 Blackbird.
"Good Morning to you as well, Madame President. I hope Bill hasn't complained too much about his speed chess defeat last night."
Hillary Clinton, President of the United States of America, was impatient. "He'll live. However, several family members of European heads of state aren't that lucky. Several of them have been poisoned and we have no idea how."
Marine Le Pen suddenly interrupted "Mathilde, elle est dans un coma. S'il vous plaît, Monseigneur Holmes, il faut la sauver!".
"As president Le Pen mentioned, one of her children was the first victim. Soon after, Belgium, Germany, Poland and Ukraine were the next to get hit. Each country has the youngest child of the current leader heavily ill. We think the Russian prime minister might be next, but we're not sure."
Sherlock sighed, then said "You're not sure? Obviously they're next. I really don't understand why you haven't made any arrests yet. It's clear who is causing this".
Holmes then explained in 3 sentences who they had to arrest, what the link between the countries is and how the link leads to the perpetrators.
He was right, obviously. But who was arrested, and why?
The poisonings are the work of a conspiracy, not one person. Each offspring was poisoned by a different person. All acting conspirators had the same profession. It's not clear what poison they used, but the symptoms point to one that needs to enter via the mouth.
As deduced by the first answer, the E40 European route is important in this question. What's also important is how many victims are mentioned or planned. the perpetrators won't head into Asia.
There are 6 victims along the E40. Put in another way, that's E406. E406 has a well-defined meaning as a code word for a food additive. It's that food additive that holds the key to the puzzle.