three kids--5/room Paris & London

I suggest you try www.travelodge.uk for London and www.accorhotels.com (x great value at Etap an dbunk type beds ensuite)for Paris.
both geared to families and have great rates when booked ahead. I have stayed at both these chains with vg results.

Would you consider renting an appartment instead of a hotel room? You can usually get more room for less money.

we are only staying a couple nights in Paris then renting a car

We also recommend Accor hotels. My wife and I and our 2 sons shared a room in the Novotel Hammersmith in London in 2007. The room was simple, clean, and nice; and hotel staff was great, especially the concierge who was a great source of information. The price of the room included a full English-style breakfast for each of us We liked the big breakfast for several reasons. First, our 2 teenage boys have healthy appetites and the Novotel breakfast was buffet-style. Second, we tended to eat a big breakfast, grab a backpack with raincoats and other gear, and leave for the day. Because we ate a large breakfast, we only needed sandwiches or something similarly light for lunch. Also, London is EXPENSIVE and my wife and I learned in our travels that breakfasts included with a room are usually a good way to cut costs.