Because the MapFragment is a child fragment, you need to use the childFragmentManager instead of SupportFragmentManager.
So you will need do something like:
MainFragment mainFragment = (MainFragment)getSupportFragmentManager.findFragmentById(R.id.content);
MapFragment mapFragment = (MapFragment) mainFragment.getChildFragmentManager().findFragmentById(R.id.map_fragment);
SupportMapFragment supportMap = (SupportMapFragment)mapFragment.getChildFragmentManager().findFragmentById(R.id.mapview);
That is untested code, but should point you in the right direction.
Something to note though: you seem to have an extra child fragment which you don't need.
The MapFragment fragment looks like its just a fragment that contains a SupportMapFragment. You should try to avoid excess child fragments if they aren't necessary. Either replace the MapFragment with the SupportMapFragment entirely, or implement the mapview yourself in your MapFragment.