Ok, so there is two ways to achieve the clean modification of the footer by mean of a child theme without loosing the custom settings :
– Save the settings in Graphene, load the child theme, load the saved settings (just have to find where the option is).
– Create the child theme with the components that are resetted : Header and Menus, for me.
Am I right ? By the way, I will do tests before ask too many questions here 🙂