What do I need to do to add my exhusband to the kids birth certificate?

What is the name of your state? Utah
My exhusband and I were together for 10 years, we have 4 children, however only 1 is biologically his. There is no father listed on any of the other childrens birth certificates, what do I need to do, or is it possible to add him to their birth certificates? We have both since remarried, but he continues to see them multiple times durring the week and assists with all financial aspects of their lives. He is the only father any of them know and all wish to have that reflected on their birth certificates (14, 12, 10) Any help would be appreciated

[Seanscott]

For the child that is biologically his, I believe it's a simple process. Just a trip to the courthouse for both of you.

For the other children - you would be committing fraud and I don't think you should try it!