By default, riders under 18 appear by first name and last initial, for example Avery S., on public pages: show results, follow pages, series standings and result cards.
It works from the rider's birth date, so add it on the rider's page. The setting is Show only the first name and last initial on public results on the rider's page: a parent can untick it for a junior rider, and an adult rider can tick it.
The show office sees full names, because it needs them to run the show.
Still stuck? Email support@dewwow.com and a person answers.