I believe school is required at 5.
My experience is there is day care for kids to about 3. Then 3,4, and 5 there is jardin infante which is like nursery school and kindergarten.
When you arrive in Sept. there will be three months left of this school year. You may want to look into some clubs and museums that have activities for pre-school aged children and focus on finding school for next Feb/March.
Good schools fill up early. Many parents start their children at 3yo at the school they want their child to attend through high school. Therefore at the good schools there is little vacancy for children who want to start at 4yo or 5 yo.
I think for starters your options depend on how long you will be here, where you will live, and budget.
With regard to where you live, will you have a car to get kids to school or do you have to stay local. If you have car do you have time and if so the patience to commute kids to school.