This is a one-day-tour we have done with a 9 month baby and an almost 4 year old toddler. We had beautiful day with the kids soaking up the beauty of Paris by strolling through the Marais and taking a boat trip on the Seine. If it´s the first time you are visiting Paris this tour gives you a pretty good overview and orientation of the city.
We started 10ish at Bastille and were back home for a nap around 5pm. The walking distance were just fine for a 3 years old child. (The baby was carried or in the stroller.) We took a hot chocolate break at Le Voltigeur and lunch at Bistro Beaubourg after checking out the street artists at Centre Pompidou and the fountain by Jean Tinguely. And with the Paris St.Germain shirt we bought for the little one he was a big hit in the bistro later.
There are way too many great places and buildings to see in the Marais, same goes for shops and bars thus just stroll around, get lost in the streets and discover your own little Paris. ( Or check out https://thewishingtable.wordpress.com/2016/01/05/a-very-hungry-caterpillar-in-the-city-of-light/ )
The boat tour will take you along of pretty much all major sights : Louvre, Tulleries Garden, Grand Palais, Eifel Tower, beautiful bridges, Musee d`Orsay, Notre Dame, Concergierie etc.
For us it was a perfect day, all parents and children were happy!