Mega Menu - Child Item & Grandchild Item positioning

Hi,

i have built the Mega Menu according to the GeneratePress documentation - everything looks good - however in this 4 column set up, i'd like the child item to span over 2 columns and the grand child items to be 4 columns.

So basically it would look like this

| Parent Item |
| Child item 1 | Child Item 2|
| GC Item (belongs to item 1) | GC item (belongs to item 1) | GC Item (belongs to item 2) | GC Item (belongs to item 2) |

I have tried the mega-menu-col-2/-3-4 but failed - any pointers? :)