I have sole legal custody of my two daughters in Washington, DC. My ex husband is entitled to supervised visitations only and he does pay child support. He however, has not seen the girls in five years and has maybe contacted them once in the last two years. My question is, "can I legally change their last name to mine since I have sole legal custody of them without having to secure his permission"?

No I don't think you can change the names without his consent because the court had already awarded him visitation rights. I don't know about the child support, but legally you are both recognized as parents on the birth certificate, apparently.

Contact an attorney