I am not so sure what the "fun" part of Shinjuku would be for an 80 y.o. and a 10 y.o., but it sure wouldn't be the OTHER side of Shinjuku station!
I would seriously doubt that the GH is worth Y20k more per night than the Penn. However, the GH does enjoy a very high room occupancy rate, I understand -- but in that case, wouldn't the new Tokyo Midtown RC have a similar structure?