Is there more cyanide inside the pit? As opposed to a whole one? What should I do it has been an hour and a half.

Plum pits contain cyanide. Cyanide gets process and eliminated like alcohol and other consumed chemicals. There isn't enough cyanide in the plum pit to significantly hurt your little one. What should you do? Nothing. There isn't really anything you can do. If you notice a change in behaviour than you should take him to the ER or Urgent care but I doubt anything bad will happen.

Ditto