The Suica & N'EX package has to be bought in the JR East Travel Service Center in T1/T2 at NRT and cannot be purchased from the ticket counter or the ticket machine. Depending on the time/day of arrival expect a long wait.

Narita Express's platform at Tokyo Station is deep down in the basement and it's quite a trek to the street level to get a taxi. With a kid and luggage in tow you might want to consider continuing to Shinagawa on N'EX to get a taxi from there instead of Tokyo. Taxi may cost up to 50% more but it will be much easier than navigating your way around Tokyo station.