Its not, you shouldn't put anything in their mouth at all. I actually lecture my students for fun on what to do if I ever go into a seizure, and I always get statements: "Shove a sock in his mouth!" "Get a spoon in there" and all kinds of other stuff. Really you just have to let the seizure takes it course, then turn them on their side.
Luckily he was a tiny kid, so when he had another seizure when the paramedics arrived we were able to restrain him easily. If it was a grown adult some of us could of ended up with black eyes or something.