If you really have to stay at Hilton Tokyo Bay, you'll save a lot of time and walking if you forget about the NEX and take a more direct route. The two best ways to do it are probably:

(1) Narita Sky Access Line from NRT to Higashi-Matsudo (40 min); change there for Musashino Line to Maihama (25 min).

(2) Limousine bus to Kaihin-Makuhari (40 min); change there for Keiyo Line to Maihama (20 min). The bus operates more frequently than the Sky Access Line so this is likely to be faster, depending upon exactly when you get out of immigration and customs. Of course, the kid will probably be more interested in the train.