Malins participated, primarily at fullback, in England winning the 2016 World Rugby Under 20 Championship hosted in Manchester. He scored tries against Scotland in the pool stage and in the semi-final against South Africa. He started at full back in the final against Ireland. The following year he was a member of the squad that completed the grand slam in the 2017 Six Nations Under 20s Championship and then featured at fly-half as England reached the final of the 2017 World Rugby Under 20 Championship, before being defeated by New Zealand. He was particularly noted by England U20 head coach Ian Vass as being a major figure in their success, acting as a calmer figure around which the team could bond. He acted as the primary place-kicker in the latter competition for the majority of his games with a 72% success rate.