Hello,
There is no need to switch to the parent theme. I found that issue was caused by
Rinvia il blocco di visualizzazione JS option of your cache plugin.
Point is that Combina file JavaScript option should be enabled if you want to use Rinvia il blocco di visualizzazione JS. In other cases, scripts have wrong order and do not work as they should. So, I enabled both options and the mobile menu works fine. Check your site now.
Regards