How should I tell my own parents that my husband has married another woman?

Choose a calm moment, give the facts briefly, say what you need from them, and ask them not to act on their own against your husband or the co-wife.
Allah commands kindness to parents (al-Isra' 17:23), and they are usually the greatest source of support. But hurt parents may react harshly, confront the husband or co-wife, or press for divorce. Suggestions: decide first what you yourself want — to continue, to talk things through, or to separate; ask them to listen and support rather than decide for you; and do not share details that would shame your husband excessively (al-Hujurat 49:12). If your parents are frail or easily distressed, it may be better to involve siblings first. Also protect the children from family quarrels. See The existing wife.