Schools years are on year of birth in France, although a few children are held back due to a late month of birth or moved forward due to early moth of birth, with co-operation of parents and school, even greater numbers are held back from primaire onwards due to failing and are obliged to "redouble" a year.
There are 3 years of maternelle or kindergarten, petite section (aged 3 on 1st January 2012 for September 2011 start), moyenne section (aged 4 on 1st January 2012 for September 2011 start) and grande section (aged 5 on 1st January 2012 for September 2011 start). A few children aged still 2 by 1st January are admitted into petite section, often only part time, depends on the school, I'm sure the discretion means it's those who are eldest who get priority for these places.
So in September 2011, your eldest would be still 5 on 1st January 2012 so would start in grande section in September 2011. Your youngest is almost definitely going to be too young to start at all in petite section in the school year September 2011 to June 2012 and would start in September 2012 in petite section.
Schools are either communal and free and you usually get told which communal school your children can attend according to your address, or Catholic and have small fees. Each region has and "académie" whic overseas schools in the area, a bit like a UK Local Education Authoirty (LEA) and hopefully has a website with lists of schools on it.
I think this is a pretty accurate response.... otherwise my apologies and someone will be along to correct me.