What you don’t know is that in the 2010’s the left wing had a agreement with the Green Party to stop all nuclear by 2050. That’s why we stop financing and maintaining a lot of nuclear plants. It turned out it was a bad idea..
Before this new law, in France kindergarden already existed and 98% of children were already attending kindergarten. (It has always been free as well) I don't think this has been done to tackle the low fertility problem in France, but more to enforce a standardization of the education of young children.