If the child is born in France:
A child born in France to foreign parents (if the mother was not a short-stay tourist) can obtain the French nationality by descent from the age of 16 and become French at the age of 18, provided they have lived in France for 5 consecutive years from the age of 11.
A change in French legislation in 2015 also allows children to acquire French nationality at 18, provided they have lived in France since the age of 6, attend a French school and have a sibling who has acquired the French nationality.
If you are a foreign parent to a a child over the age of 13 who has lived in France since the age of 8, you can request the French nationality on their behalf.