I have sole custody of my 8 yr old twins... the bio father is not on the birthcertificate but... I had given the last name.. (why I don't know) I think when we established paternity it happened.. but my sons and I both want them to have my last name.. how would I go about doing this and do I need the bio's permission since like I said I have sole custody and only my name is on birthcertificate

I don't know if this applies in your state, but here's two sites I found:

http://www.health.state.mn.us/divs/chs/osr/amend.html

http://www.babycenter.com/0_how-to-change-your-childs-name_3641846.bc