Do I need a lawyer to change last name of my kids?

2 scenarios...

#1:
I have a 10 yr old son with my ex-husband. I have full custody. Child spt enforcement closed my case because they could not keep up with my ex's job hopping so I haven't received any child support in almost 2 years and he has had no contact with my son in 8 years.

#2:
I have an 8 yr old daughter with an ex-boyfriend. He left me when I was 3 months pregnant, haven't spoken to him since February of 2001, which was 2 months before my daughter was born. There is no court order involved, no child spt, he never showed up for paternity testing. She does not have her biological father's last name, but the last name that I had when she was born.

I remarried nearly 7 years ago. My 10 yr old knows my husband as his dad because he no longer remembers his biological dad. My 8 yr old daughter has known my husband as her dad since she was 9 months old.

My children do not know the truth, they have been told that there dad (my current husband) and I were not married when they were born and that is why they do not have his last name (which is the truth). they are both wanting there last name changed. Can we do this on our own? Without a lawyer? Do I need to file a notice in the paper? my local paper? or the paper of the biological dad's last known address? I'm only seeking a name change, not adoption.