There are no passport or visa peculiarities for people traveling to Japan; children need the same documentation as adults. Though it may not be required, it's a good idea to have documentation on hand regarding the relationship between you and the child as well as permission for the child's travel from the parent(s) or legal guardian if they are not present. Having such documentation with you will only facilitate your entry and departure.
Visitors from the United States with passports valid for more than 90 days from their time of arrival can stay in Japan for 90 days; no visa is required so long as your visit does not include paid activities.
