you could always use
to inform a object that it needs to be recomputed.
General question: If your object does not have a link to the sketch, how does it know which sketch to use for rebuilding the polygons? Or if the polygons are individual objects, how do they know how to rebuild if they don't link to the sketch? You should explain your documen tstructure a bit more detailed.
What I could imagine is that you build "dump" polygons from the sketches which as objects don't know how they have been created. Is that correct? If this is the case, then the objects themself don't have any information how to be rebuild themself, and it is not enough to inform them of the sketch change, as they don't know which sketch to use for rebuild. (if they would know it as they have a link to sketch than freecad auto recompute would work). In this case you could keep the dependencie information somewhere else and use callbacks to rebuild. Have a look at Document Observers
which you can setup and which inform you about changes to objects. You can than detect any change to the sketch, see in your dependency tracking if it relates to the polygons and rebuild polygons.