Is there really a JR counter at 1F in Terminal 1 at NRT? They redid the travel center in the basement earlier this year and there are huge lines there in the afternoon since it happened. People with passes have to go there, I believe. It wasn't that busy before, even though my experience partly overlaps the post-Fukushima era when there fewer tourists.

Since you are not saving any money with a JR Pass you should consider just paying as you go.

The Hikari train is slower but also much less frequent. The Nozomi leaves literally like every 5 minutes vs. 30 min for Hikari. Then you don't have to wait in the long line at NRT to process your pass. You could just buy a Suica-NEX to Shinagawa and book the next Nozomi. Allow 10 min transfer with toddlers at Shinagawa.

Actually though, if you arrive at 10:30 am then you should be ok at NRT. Things are a lot quieter at that time.

You definitely don't want to take the Metro from Tokyo eki to Roppongi. It's probably less than 5 km so cab fare is reasonable; you would need to change trains.

Then when going back to NRT from Roppongi it will be easier for you by bus. If you pay as you go a one way Suica-NEX, which gives you a Suica with 2000 yen value (500 deposit plus 1500 for all public transit in Kanto region) for 3500 yen.