My daughter wants my husband to adopt her and so do him and I.
First of all, your daughter is 6 years old so her "wants" are legally irrelevant.
Second, you can tell your husband that it doesn't take a piece of paper for him to become her father. My daughter was 6 years old when I married her mother. Never could get consent to adopt. But guess what, she is 40 years old now and I am still her father and she is still my daughter.
As for the legal stuff, your option is to hire a lawyer who can file for the adoption in court and see if you and your husband can convince a judge to order the adoption over the objections of the father. Count on paying many thousands for that option with no guarantee of success.