The child fares are for kids that are under 12. They pay 50% of the adult fare.

When I took my 12 year old daughter with me to Tokyo last year, we went to the new rail ticket window in the arrivals area of T1. The nice young ladies at the window heard that my daughter was 12 and told me that 11 year old pay half. I told them that I didn't want to lie to them about her age. But, if they thought that she was 11, well, I'd have to go with it! They promptly presented 1 adult ticket for me and a half price one for her.

On the way back to NRT, her youthful fare saved us enough for lunch at the NRT T1 MacDonalds (somewhere I NEVER would have expected to choose to dine). Result: The world's ULTIMATE Filet-o-fish sandwich!!! Fantastic. If only I had additional yen to try the teriaki burger, too.