[Interregional transportation]
Regarding Question #2:
However I checked Narita to Osaka flight, and it seems convenient and cheaper option.I am not sure whether the 7-year-old boy would prefer a Shinkansen super-express ride or a domestic flight, but if you fly from NRT to KIX, a Japan Rail Pass is not likely to save much cost for your current itinerary unless you use JR-East in your Nikko trip.
There is also Osaka Airport [ITM, Itami Airport] in the same Kansai Region as Kansai Airport [KIX] is, but a railroad route to Kyoto Station is simpler if you choose KIX.
To travel from Kyoto City to Tokyo Metropolis, Shinkansen is recommendable.
It is a roundabout to take trains to KIX or ITM and from NRT or HND while a direct Shinkansen train is available.
[Flights, air passes and stations]
Singapore Airlines SQ 5904 / United Airlines UA 9674 will be operated by All Nippon Airways [NH, ANA]. These three companies use NRT Terminal 1: South Wing, and they belong to Star Alliance.
If you come to NRT by any of these two international flights and travel to KIX by an ANA domestic flight in October, you can choose a Star Alliance Japan Airpass.
http://www.japan-guide.com/e/e2364.htmlKansai International Airport [KIX] has two terminals.
Railroad stations are in KIX Terminal 1, but not in KIX Terminal 2.
Among so-called low-cost carriers, Jetstar is using KIX Terminal 1, while Peach Aviation is using KIX Terminal 2.
Narita International Airport [NRT] has two terminals.
Railroad operators JR-East and Keisei have Narita Airport Station(s) in NRT Terminal 1 and Airport Terminal 2 Station(s) in NRT Terminal 2.
When you head to NRT by rail, please note that Narita Station and Keisei Narita Station are located outside the airport.