How do I go about changing my kids last name to mine? Their father and ii were never married but he is on the birth certificate but no custody has been established. I have had them 24/7 and he barely sees them. They are 4 and 2.

How old is the child?

If the child is less than a year old, you should be able to change their last name without too much problem.
If the child is over a year old you have to get permission from the father before changing the child's last name.

I want to add my last name to my two older kids birth certificate seeing how I haven't seen or heard from their biological father in over eight years can I do this and how can I do it