Definitely go to Paris if you can in summer! We went last year with our boy-girl twins a few months before their 3rd birthdays. They'll love Paris Plage, with sand, lots of kid stuff and even trampolines down on the Right Bank of the Seine. Boat ride was a big hit and just wandering the streets separately with them or together was also much fun. Discovered a very cool little playground in the Places des Vosges that I'd not noticed on earlier trips.
As for price, I got a somewhat reasonable hotel deal through expedia, but I wouldn't call it cheap. Maybe it was $130 a night or so ... but kids under 4 go free on French rail, including the TGV, which is dirt cheap when you book in advance online.
You could also go in for Le Camping ... the French do it way better than KOA. At the campground they'll have a pool, cafe/bar, fresh croissants in the morning, etc. Before we stopped in Paris for 3 nights, we camped for 10 days in Provence, near Sanary-sur-Mer, which was a blast.