Lisa - the "ideal" month for such a cruise is June, to encompass midsummer's night eve (I believe around June 24th or so).
There are basically two itineraries - only the fjords and fjords/North Cape. Speaking for myself, the ports that I recommend (not necessarily in order) are Bergen, Geiranger (most of the photos you'll see of the fjords taken from on high are from Mt. Dalsnibba overlooking Geiranger and the Geirangerfjord), Flaam, Trondheim, Tromso, Honningsvaag which is the port for North Cape. I'm rather confident that you and your family would find the motorcoach ride from Honningsvaag to North Cape to be most interesting - tundra-like terrain and herds of reindeer scattered about. The narrative from your tour guide adds to the enjoyment. Tromso is Norway's largest city north of the Arctic Circle and filled with many interesting museums and a prestigious university - its medical school did pioneering studies on the phenomenon of depression in darkness. Trondheim, Norway's third largest city, is home to the Ringve Museum housing wonderful musical instruments from around the world. Bergen, second largest city in Norway, is a beautiful medieval city rich in history from the Hansa League days. I love simply to walk around the center of town surrounding the port area.
All of my cruises to this part of the world have been on Crystal - and the enjoyment was made even greater because many of the Officers are Norwegian. Wending in and out of the fjords, they have a narrator giving the history of the area - and they usually just happen to find Edvard Grieg's music to pipe through the P.A. system during the narrative. Then there are festivities on deck (weather permitting) as we cruise off of North Cape at midnight - it's quite a party.
None of the traditional cruise lines frown upon having well-behaved children on board - I'm sure your son will be fascinated by the sights. HAL, for one, offers this itinerary.