Tillysmummy, we took our 6 yr old to Paris last year and had a great time. Got a book called Around Paris with kids by Fodors. Just looked in for things to suit you:
Jardin D' Acclimiatation
Bois de Boulogne 16e
Metro; Porte Maillot, Sablons
Open June-Sept daily 10-7
Paris's oldest amusement park, all recently renovated; mini farm, riverboat ride, fairground type games, Maison Enchantee has things for all ages, a foam room for babies age 9 months up!!
Fun way to arrive is to take Le Petit Train from the Porte Maillot (at the edge of the Bois de Boulogne) to the park entrance. It runs every 15 mins on Wed, Sat and Sun and every day in hols.
Park contains snack bars and 3 restaurants.
We had a stop for weary legs (after Sacre Couer) at the big store Galeries Lafayette. Great toy department, loos, an indoors McDonalds. Rue Boulevard Haussmann.
We found the buses really easy to get the hang of, and spent most of our time on them as opposed to the Metro.
Instead of a Seine river cruise, too long for littlie I think, you can buy a River Bus ticket and get on and off when you want to.
Have a lovely time, there are lots of parks and fountains etc, where you can stop off and let her let off steam!