OP, I listed a bunch of questions, but then I realized I had done the math wrong and assumed your grandfather was an adult during the occupation. I've since deleted that post, but am compiling another list directed towards a child's experience. I will update this comment later this evening with those questions.

OK, here is a different list of questions that would be better directed towards the experience of a child.

1. What’s your earliest memory of the invasion? How did your parents react?
2. What were your parents’ political affiliations, if any?
3. What was your confession?
4. What kind of school did you go to? Public? Bijzonder?
5. What is your fondest memory of a German soldier?
6. What is your least fond memory of a German soldier?
7. What changes did you experience in the school immediately after the invasion?
8. Did you have any teachers that were Jewish? Did you know what had happened to them once they were removed from the schools?
9. What do you remember about the bombing of Rotterdam? How significantly did it affect you and your family? Your friends?
10. Did you have any friends or classmates who were members of German organizations, such as the Hitler Youth? What about Dutch Nazi organizations, such as the Jeugdstorm?
11. What was your opinion of such people? How did that opinion change during the course of the occupation?
12. Did you ever get to listen to Radio Oranje? How so?
13. Were you aware of any resistance activity in your neighborhood? Please describe …
14. Do you recall any of the curriculum changes that the occupation regime instituted?
15. Did you notice any difference between teachers who were pro-occupation and those who were against it, in terms of teaching content?
16. Were there a lot of changes in teachers during the occupation? What about the school director? Do you know why those changes occurred?
17. Did you have any Jewish classmates or friends? What happened to them? Did you know at the time or did you later learn of their fate?
18. Assuming he wasn’t a member of and NSB affiliated organization, how did he and his classmates treat such children? If he was, how did his classmates treat him?
19. Really, what was it like riding on wooden wheeled bicycles?
20. What was your food situation during the war? Do you recall your parents having any difficulties finding food? Was one period worse than others? When? Why, near as you remember?
21. What if any connection did he or his parents have to the resistance? Did you hear rumors about the resistance?
22. On a personal level, who treated you better, German soldiers or Allied soldiers? How so?
23. What is the first thing you did after learning of the Allied invasion in Normandy? What about once your town was liberated?
24. Were you aware of the February strike? What did you know about it? Did you know anyone who participated? What happened to them?
25. Were you ever worried about having to do labor service in Germany (he would have been too young to actually do it, most likely, but he might have been aware that it was a possibility in his future).
26. What was the thing you remember most fondly about the occupation generally?
27. What was the worst thing you remember about the occupation generally?
28. Do you remember Prince Bernhard’s birthday in 1940? What happened? How did you, your family, and friends participate (if at all)? What were the reactions of you, your family, and friends?
29. Do you remember the general railway strike in 1944? What happened? How did you, your family, and friends's parents participate in any way (if at all)? What were the reactions of you, your family, and friends?
30. What is, in your opinion, the greatest lesson someone can learn from the occupation?
31. In what ways did your childhood experience influence your adult life?
32. How do you feel about Germans today?

Ok, this is all for now. As before, I'd be very interested in reading what you eventually write. And again, make sure and use this time, first and foremost, to bond with your gramps. College is important, but getting to know your family is more so.

My father was with the Underground after he left hiding. ask him how his neighbors/friends/family felt about the resistance to the occupying forces and those in the community dealt with collaborators. How did he and his family cope? with the blitzkrieg destroying the plant, was there damage to the city? was his home hit? Did he resist, and if so, how?

(my father was interviewed for Spielberg's Shoah)

I don't know how personal you'd like to get with the questions, but you could ask him how he, as a young boy, processed and perceived what was happening around him. How did he rationalize the abduction and internment of any potential friends or neighbours? When did he realize that things in his community were going to be rapidly changing? Today, does he feel guilty because he was able to be escorted while others around him were being murdered or persecuted?

A couple of related points, if I may. My work editing documentaries have caused me to watch thousands of hours of interview.
Don't be afraid to let this evolve as a conversation. If something comes up that interests you, follow the thought before returning to the next question. Allow one anecdote to lead to another, because oftentimes the most interesting comment made isn't in direct response to a prepared question.
Don't be concerned when you get a 'nothing' answer from several of your queries. That's normal, and nothing to be concerned about. Watch out for the difference between opinion, and memory. You're primarily after what it was like at the time, and only secondarily what he thinks about it now. Your goal is to assist him back to his past, and for him to re-experience the sights smells and textures of his youth. The way memory works is that we remember things most accurately when we haven't thought of them since the event. Help him do that, and you'll get something truly worthwhile for you both.

Oh, and test your recording kit before you begin! Enjoy.

I've heard Canadians were quite well regarded in Holland after liberation. I would be curious to hear if he remembers any interactions with Canadian soldiers, positive or negative.