What about a Sling Box for the set that is not used much, or let the kids if they have tablets, use the Sling Box app to watch shows on the tablets.
Or just use two boxes, and when the kids want to watch, but no one is watching the other set, use a HDMI over Ethernet setup, that uses Balun's that allow IR changing at the box, from the TV.
Just a couple of ideas for you. My son watches more Netflix streaming, or plays his PS3, then he really watches tv. I am beginning to rethink why I have 2 C31/700's. One of them hardly ever gets used, and that is the one in the Master Bedroom. I should have just gone with two myself, and done the HDMI over Ethernet, with the Balun's I mentioned before. At least that would save me around $10/month.
There is also the "Go" apps for the Roku. Do not know how old your kids are. But if one plays video games and one watches Cartoons or kids movies, there is also the Chromecast or Roku, that they could use to stream to the tv, if you only go with two boxes.