Yes, a husband, wife, and child can together form a Hindu Undivided Family (HUF) and open a HUF demat account. In this setup, the husband will act as the Karta of the HUF and will also be considered the first coparcener. The child will be the second coparcener, gaining rights in the HUF by birth. The wife, although not a coparcener, will be recognized as a member of the HUF. With this valid structure in place, the family can open a HUF demat account for managing joint investments and ancestral assets.