Le Meridien Montparnasse--two queen beds and lovely large rooms, rates are quite good.

To help future searchers of this topic, I'm happy to report that my sister and her family were very pleased with their stay at the Hotel du Danube. They rented the two room apartment which worked well for them and was quite reasonable at 250 E. Thanks, avalon, for the suggestion.
http://www.hoteldanube.fr/apartments.php.

I have to beat a dead pony here, if not an actual dead horse.

Young children -- ten ear olds, certainly -- regularly cross Paris by public bus and Metro to go to and from school. By themselves. No mom. No nanny. No schoolbus monitor. Thus it would seem odd to a French person or long-term resident that young teenagers could not share a room of their own.

Well I must admit I don't get stuffing 4 people into one room unless the kids are small.

Our first trip the girls were 11 and 14 - certainly old enough to stay in a room by themselves. After all they had been going to camp for years and never wandered out into the woods to be eaten by bears. And if they didn't know enough to behave properly - well then it's time for a good life lesson (as in consideration for other people).

Perhaps it's because they were coping with NYC themselves - as in going to school and activities with friends and taking the subway etc (which I had also done since I was 12, despite the fact that I lived in the suburbs) - but if kids that age don't know enough not to leave the hotel and wander around at night - they are incredibly naive and the parents need to really step up their act.