We went in September and had a great time with our son.
Sacre Couer, up all the twisty steps, if babe doesnt object to being carried, great views. Little land train that goes round the area from the front. All the artists paint nearby and sell their wares. If you walk down all the front steps there is a little playground.
We then walked down towards Galerie Lafayette, its on all the maps (I think they sponsor the paris maps that get given out) big dept storem fantastic toy dept, our son loved all the computer games out to try, big loos and an inside McDonalds on the same floor.
We really got into using the buses, they were ever so easy, when we arrived in Paris we got a 3 day tube and bus ticket. So after that never had to bother with queuing etc. Big queues for metro when we arrived at Gare du Nord.
If you go to The Louvre, go down the escalators inside the glass pyramid, and there is a shopping centre down there, and a food hall with ALL different foods, we had some great crepes there.
Got to do Eiffel Tower of course! Get the metro to Trocadero (I think) dont have map handy, walk round the corner and you have a FANTASTIC view of it, you walk down lots of steps, big wide/shallow ones, fine with buggy, past the fountains, over the Seine. Go early!! The queues are BIG. You could avoid queues by walking to the first florr (ha-ha!) or booking a table on the first floors restaurant for lunch, good food apparently, and they have their own separate lift on one of Eiffels 'legs'!
From Eiffel you can just walk to the Bateuax Parisiens for the boat tour of the seine, might be a mite long for toddler.
After visiting Notre Dame, you can walk to Pompidou, wacky sculptures on a pond outside, nice restaurants alongside. Usually lots of entertainers outside the Pompidou entertaining the crowds.
Cant think of much else at the mo, we did enjoy just wandering with the map and finding places.
We stayed in the 6e, St Germain area, quick and easy to get to from Gare du Nord, wouldnt recommend the hotel, but there are so many. Nice area, lots of cafes/restaurants, book shops, felt very safe etc. Short walk to Seine.
Janus you'll have a wonderful time, its a great city and there is alot to catch the attention of a toddler.
Willow2, how romantic, Paris in the Spring! Have a great time.