No update is made automatically, we don’t force notifications in WordPress. So you can be notified when we change it in Themeforest.
The child theme is a way to extend the parent theme, so it don’t exist any update in the child theme.
But if you have a modified file in the child theme and that original file in the parent theme is updated, the correspondent file in the child theme should be compared with the parent theme file so someone can update any major change that can break the functionality.
-Rui