At 7 years old, when Greenberg's younger sister Becca was in dance class, he got the opportunity to appear in the lead role in the Omaha Ballet production of The Nutcracker. He toured with the show for two months and joined a children's theater company in Omaha. At age 12, Greenberg and his family moved to St. Louis, Missouri. Shortly thereafter, he got his first national role as the kid in the Cookie Crisp cereal commercial. As time went on, he was offered more acting opportunities.