We are in the middle of doing this with 3 of our grandkids that we have legal custody of. It's a long process for us but we'll get it done. The mom's have signed off and since one dad is dead and the other in jail we're told it shouldn't take much longer. (I hope)
I took my maiden name back after my divorce and got my children's last name changed to my maiden name. I had to get my ex to sign off, agreeing to it. Technically, if he hadn't signed off, I could have still gone to court and argued for it, and possibly convinced a judge to grant it anyway, but the chances were much slimmer that way.