The general consensus is that a second subwoofer will help with the frequency response and even things one. As stated above, the current subwoofer is already more then enough to topple small children in that size room, but adding a second will help with the response and make the sweet spot larger (Without resorting to EQ and electronic trickery).
Acu has a good point though, I don't like randomly throwing gear into my system (not that I can afford to either). I like to think it through, how not only the gear is going to immediately fit, but how it will fit down the road. If it's a good match now, but not sensible later on, I usually hold off.
Besides, Acu is pretty much the spitting image of someone that doesn't just randomly stock up on stereo components and speakers.
SheepStar