First, be sure that there is plenty of air flow for that new receiver. If you have an open back and front, with about 2" of space above the receiver, you will probably be OK. Otherwise look into getting some sort of heat removal system, such as:
Use the 70s, as they are probably more efficient than the 40s. Yeah, it's counter-intuitive, but the larger speaker is typically the more efficient speaker. The Marantz has a true 50 watts, it's not marketing fantasy, as so many receivers advertise (100 watts! Yeah, right!
). It will be plenty of power as you are not sitting that far away, and with the kid, you won't be cranking it up to deafening levels, anyway.
Use the center channel if you have any folks sitting on the side couch. Having a center will maintain the sense that the sound is coming from the TV, even when seated off to the side.
If those rear surrounds are already wired and ready to go, no reason not to try them out and see if they add anything to the movie experience.
Lastly, that sub is the weak link in your system, so look into replacing it at some point in the future, when funds are not so tight.