Hey olollo,
the child theme’s portfolio grid page should work even when the child theme only has the required “style.css” file – I’ve just double-checked this on the theme demo website. You might want to update to the latest version of Ikebana (3.4 at the moment, is available from the “Downloads” tab of your ThemeForest account) and try once more; alternatively – yes, the “header-functions.php” file is the one containing the script that initializes the portfolio grid, other external script files are referenced in the “functions.php” and should load automatically.
Let me know if you keep having problems with the child theme!
Sincerely,
Andrii / Satori Studio