If you were under the age of 18 when the incident happened then you can answer No to the quesiton about arrests and convictions.
This article explores the issue.
http://britishexpats.com/articles/uk-police-cautionswarnings-and-us-immigration-law/
If it was me then I would request a copy of my police record. If the incident does not appear at all in the polcie records then it probably means the police did not make a report of the incident at the time and it is a nothing. You are probably safe answering No even if you were over the age of 18 if that is the case.