Probles with Upfront child theme


we are having some issues with an upfront based child theme "Panino"... usually child themes shouldn't be updated so that the changes made to code aren't deleted... but after the update of Panino our code gets removed.

As i know it's not possible to make a child theme of a child theme (or grandchild theme). So where should we put our custom code (functions.php addons, javascripts, css,...)

Kind regards