I.M Meen may be eccentric, keeping smart children locked up in cells and whatnot, but the sheer horror in this is the fact that Meen has probably been kidnapping children for quite a while, which means that there were more children before you who were locked up and it's very likely that they never escaped.
Confirmed. You have to free other children, meaning that until Gnorris helped you, they never escaped.
Why didn't Gnorris help the other kids to prevent you from having to save them?
Probably because I.M. Meen would have punished him severely and, more importantly, he wouldn't have gotten them anywhere. Your character was probably Gnorris's best chance at actually succeeding at thwarting I.M. Meen.