@Swissgreys why would you avoid plums? I've given a very small piece to my boys once. I thought it was ok as a treat am I wrong?There is some uncertainty around the suitability of cherry leaves and twigs.
In general I would say it is safer to avoid any things from trees which grow fruit with stones (cherry, plum, etc), however things like apple and pear are usually well recived and are safe to give piggies.
No you aren't wrong - a small amount of flesh from a plum will not harm your boys.