I accidentally ate non-kosher food, what should I do to atone for the act?

My brother received a gift basket from a child in his class in preparation for Purim, and there was a package of candies with a Hebrew label like others in the basket. We thought it was kosher, believing that Jews wouldn’t include non-kosher products in a gift basket for others, and because of the Hebrew label, we assumed it had kashrut certification. After he ate the candy and I hadn’t swallowed mine yet, I noticed it did not indicate that it was kosher, and now the thought that we probably ate non-kosher food is bothering us. I read that we should start keeping Shabbat, but we are traditional and have been observing Shabbat and holidays for several years. I wanted to know what we can do to atone for the act?

Thank you very much