James - Thanks for the suggestion - I'll look into the Lasko when making a purchase.
Posthumane - Thanks for the overview of the different types, which makes sense. Based on your description I actually think the oil-filled type is what we need - it's basically going to be on a very low setting all through the night, to keep toddler's room temp at a reasonable level (reasonable is of course a matter of opinion, and differs in our household... see below :) Regarding electric versus gas - we are renting so upgrading is not our decision, although I think gas heat would save us a ton. When we consider buying a place it will definitely be a consideration (It's strange - I live in an area with hydropower so it seems like it shouldn't cost so much to heat our house, but it does.)
Bogart and Perpetual student suggested heating the babies better, and that led me to look into warmer sleepwear. THe problem is that he just won't stay under his blankets and doesn't seem to know to pull them up if he's cold (hard to really know what he knows...) A canopy wouldn't work for our toddler, as he'd assume it was an added play structure to climb on/tear down. We also have a newborn who sleeps in our room. It's harder to bundle them up because more blankets become a safety risk, but I think a sleep sack over PJs would do a lot to insulate.
A big issue is that my wife, who is quite on-board with Mustacianism, generally, has a sort of biological, maternal opposition to removing a heater if it gets at all cold at night (and we're talking now, August and September in the PNW) I'd like to get a thermometer and measure just how "cold" it gets, but the reality is that I don't think she'll budge on the space heater, so warm clothing becomes a moot point; or rather, we have to start worrying about his PJs being too warm...
Gooki - I'd love a heat pump but it's not doable where we live. Maybe when we move!
CG - Thanks for the tip - will look into that one.
Thanks again, all. Stay tuned for an upcoming post when I'll ask about diaper drying in humid conditions :)