From the main train station it’s a tram ride to the rally grounds and documentation center. Too far to walk.

You can walk around the grounds at no cost but there is a modest admission fee for the museum which is very informative and gives you a much broader appreciation of the layout of the grounds and its significance to the strategic thinking of the party in power.

Depends how you define 'distressing' and how mature the child. I would advise parental guidance. As long as you can explain and describe the history and the child is able to take this on board then museums such as this are absolutely essential for their learning and yours. We took our children to Auchwitz aged 10 and 12. They were no more upset by the experience than me. They are not scarred by the experience but we still talk about it 20 years later it's something none of us will ever forget. Children are incredibly robust.

Not that I noticed, but the staff were very helpful

There definitely are toilets in the museum adjacent to the rally grounds. The distance beween the two sites is no longer than 5-10 minutes easy walk.

Definitely the city tour. Enjoy!

We did go on the tour and found it very interesting. Our guide was a professor from the university and was well versed in the history of WW II. I am not sure about the audio guides. Our tour was arranged through our cruise line.

There is an entrance fee for the Documentation Center, but to see the rally grounds themselves, there is no fee...

Tram (S Bahn) 9 from central station to doku zentrum. €5 entry to the museum. Tram cost €2.60 each way. Hope that helps.

It really depends on how interested you are and how much you want to see. You could just view the rallying grounds and the outside of the buildings in about half an hour. Or you could view that stuff more in depth plus the museums and easily spend half a day.