I strongly suggest that you contact the Japanese Embassy or Consulate in your country to ask these questions.
One thing that jumps out at me is that you say you will have a letter from only one of the parents. In many countries, permission from both parents is required.
If the child in this case were older, I'd think it would not be too difficult, although I agree with the suggestion to have a power of attorney for health care decisions. For an 11 year-old, presumably staying in hotel rooms with an unrelated adult, all sorts of alarms seem quite likely.