Does the Principal ever visit his granddaughter? In episode 26 of Nichijou, it's revealed that the Principal of the school...


   ...is Mr. Shinonome.


However, the relationship between him and


   his supposed granddaughter Hakase


is never elaborated on in the anime.

In the manga, is it ever stated if he at least visits her?
Answering the question directly, there are no occurrences where the headmaster visits Hakase in any media, but they have met once.

In Nichijou no Shousetsu light novel, there's a chapter called "Nichijou no Mukashibanashi" (Everyday's Folklore) telling the story of Momotaro with Nichijou's characters. Hakase acted as Momotaro, while the headmaster's role was the dog. Here's the relevant script:


  さて、さるをお供にしたももたろうが、元気良く鬼が島に歩いて行くと、その途中で、今度は犬が現れました。
  「……!」
  「……!」
  しかも、戦っています。
  しかと、戦っています。
  いぬ役の校長先生が、役をほっぽり出して、しかと戦っています。校長先生、出番ですよー!
  [中略]
  「な、なんでしかと戦っているのー!?」
  このおじさんが校長先生だと知らないももたろうは、あわてるばかり。
  たしかに、街中でいぬとしかが戦っていたら、誰だってびっくりします。


Translation:


  And so, the monkey that befriended Momotaro and the vigorous demon are walking toward the island. On the way, this time they see a dog.
  "......!"
  "......!"
  Moreover, he is fighting.
  He is fighting with a deer.
  The headmaster whose role is the dog abandoned his role and is fighting with the deer. Headmaster, it's your turn now~!
  [...]
  "Wh-, why are you fighting with the deer!?"
  Momotaro, who doesn't know that this old man (ojisan) is the headmaster, is only confused.
  Certainly, everyone will be surprised seeing a dog and deer fighting each other in the middle of a road.


P.S. The headmaster's name was only mentioned in the anime.



Source: Yahoo! Chiebukuro