Finding ancestors/Indonesian roots

You're going to have a hard time with this one.
With no names, only a guess as to where they were from, and at a time were few records were taken, much less kept over time, it's likely to be extremely difficult to impossible to find out anything about them.

You're absolutely correct. I'm just taking every chance I get to ask the question. You never know when I come across somebody who knows where or how to dig... :-)    Greetings from Belgium

If you knew the village where they lived in you might drop extremely lucky and find someone who remembers their name, maybe even a few details, but your chances are very slim.

Digging for ancestral records is never an easy task, but, it's not necessarily a "Mission Impossible."

The Dutch were remarkable in keeping records, and so long as copies of those records made it to the Hague...and not destroyed during the almost four years of Japanese occupation...there is always hope.

As Fred pointed out, an initial start with the kampung where they lived would be an excellent start, but keep in mind that those records are almost certainly to be via memory, and not any written, or documented records.

Depending on when your grandparents went to Holland / Netherlands could well dictate any records at all, if between 1939-45 it’s quite likely these records had been destroyed by the Dutch themselves, this was purely to protect the Jewish community along with other groups
Similar actions also happened in many parts of south east Asia occupied by none friendly forces (Japanese in this case ) during WW2 if after these dates you have a chance