You can use > character and numbers to select a submenu entry as default.
0 refers to first menu entry in grub. If parent of subentries starts from index 1 and you want to boot from third submenu entry as default (so that index number is 2), you need to change GRUB_DEFAULT variable as below:
GRUB_DEFAULT="1>2"
Please note that, there are no space characters before or after >
But, it is hard to describe submenu entry selection with numbers. There is another way, you can also use menu names like below:
GRUB_DEFAULT="Advanced options for Debian GNU/Linux>Debian GNU/Linux, with Linux 3.18.25.jessiemptcp"
After that, you need to run sudo update-grub command as well.