Changing names

I have 2 kids with me full time from previous marriage they have had no contact with their bio father in over 4 years - his choosing! I have been remarried for 8years and we have a little one together. My older kids call my now husband dad and view us as a family of 5 unit. They have both requested their names be changed to our last name as they don’t want to be associated with bio father - 100% their choice! I have put if off for a while as I don’t want them to have regrets about 18 months ago we changed their “preferred name” to our surname but they have been asking to change it all the way so their report cards, passports etc have same surname as rest of family. How do I go about this? As I said no contact with bio father in over 4 years and he has a hefty arrears amount with CS agency that I don’t expect to ever see. I’m not bothered by the child support I can support my kids with all they need if he ever does cough up the money it will be going straight into the kids accounts for when they are older. I just want kids to be happy and they currently get very angry/upset when they are referred to as their legal surname.