Bonjour Balhamite!
How exciting!
I am a French mum from Paris and we've moved to 2 different countries since our children were born.
There are many English speaking families in Paris so, I would not worry about being isolated. Although, this might mean making some efforts at the beginning but there seems to be some natural solidarity once you are away from home.
I have just found a website for playgroups and there are more, I'm sure:
http://www.meetup.com/English-Speaking-Mums-of-Paris/
As far as areas similar to Balham, the outskirts or the western suburbs would compare. I would look at Montreuil (Eastern side of Paris) also. There are less parks and the city is less spread than London.
Paris is now the second most expensive city in the world, something to keep in mind when your husband negotiates his relocation...
As far as schools, children start when they are 3 and it's free (and great!). It's amazing how easy they can pick up a language and there might be in a class with other bilingual kids as there are many American and British families in Paris.
There are bilingual schools but I'm not familiar with them. However, I came across this site:
http://www.expatica.com/fr/education/hi ... 10469.html
I hope it helps! Any questions, feel free to write back.
A French tutor and mum in London!