Things that your child may wish to report in Animal Crossing (though highly unlikely) include: an inappropriate player or island name, a bulletin board post, custom design, a chat comment or disruptive player behaviour.
If you wish to report on any of these aspects, you will need to navigate to the appropriate area that contains the content you wish to report. Select the content or press the (-) button to bring up the in-game menu. Highlight the specific content you wish to report, if prompted, then select ‘Report’. Select the reason for your report, then select ‘Next’. Add any additional details, then select ‘Report’.