Translated as "where goes grandfather" which I admit sounds awkward, but should be accepted.
In case anyone else is tempted to be a big show-off, I can confirm that Duo does not accept "Whither goes grandfather", and (John 13:36 notwithstanding) it probably shouldn't.
Vart implies direction, whereas var implies place.
Vart går farfar? = Where to is grandpa going?
Var är hatten? = Where's the hat?
In spoken or informal contexts they're often mixed though, so you might well hear one mean the other and vice versa. However, you should get to know the difference as it might come in handy.
This is why I think "To where is grandpa going?" should be accepted as an answer.
I got lost about family names, so I found this video: https://www.youtube.com/watch?v=8fspzcZjtNc
It explains the differences between farmor/mormor, farfar/morfar etc.
I think there should be an explanation about this particularity of the Swedish language. I was told that Hungarian also distinguishes 4 forms of saying 'brother/sister', that's a lit bit unusual for us Western people.
Unusual to us western people? Hungary is technically in the west... Or at least the eastern part of the west...
Your answer would be 'VAR går farfar', ie. he is walking around in a place. VART implies walking in a direction, whereto.
I put "Where is your grandfather going?" and it said that it was referring to the speaker's grandfather, rather than the person they are speaking to. How do we know whose grandfather is being referred to? Is it just the context of the situation?
Just out of curiosity. Why the "Where is paternal grandfather going?" phrase is not accepted?