If you're anywhere near the Cities, I have a portable seat you could test drive to see... I've not really used it with my kids - floor seems OK to them OR they sit on top of a pack.
Main thing with the strap on seats is that they sit best at gunwale level (the one I have anyway), and if you adjust it lower to lower center of gravity, the side clamps are at the wrong angle and slip off.
Well, that's my memory anyway - haven't used it with many KINDS of canoe and haven't used it in years. You are welcome to try it if you are nearby.
No other ideas or options except if you are talking about paddling near home, what about bringing a small stool inside the canoe? Like a step stool or the kind you put your feet on under a desk (sometimes called nursing foot stools)?
foot stools The hard plastic drop in ones that I've seen in pics might be wide enough for both kids - could be an advantage.