If you rent our Montmartre appartment- studioloft, you have enough space, they can play as they like, make noise without disturbing the neighbourgs, eat what they like and feel at home.
If they don't have a metro in their homecity, the metro itself is an event.
Another way of transportation can be a boat on the river. The Batobus has 8 stops and brings you along the Seine to all the big attractions : the Eiffeltower, the Notre-Dame, the Louvre, etc
The streets and squares around Centre Pompidou have a lot of streetartists, what kids love. The Centre itself is colourfull, the fountain close to it has statues of Nikki the Saint-Phalle.A visit to Montmartre is always charming, maybe you're intrested in a picture of your beloved-ones. Take your time and choose an artists at Place du Tertre.
Show them the Egyptian Obelisk at place de la Concorde, before heading to the Tuileries with the lake where they can hire a small wooden boat. Maybe you can try a visit to the Louvre (free admission for children) . Most of the children like the statues of the Greeks, Egyptians, Romans.
When your children are still into playgrounds, there is a nice one at Jardin de Luxembourg and a lot of smaller ones all over the city at the little squares ( nearby : square d'Anvers)