I was on the kid side. My mom got remarried when I was three and when I was almost five she found out she was pregnant and talked to both me and my dad about it. He agreed to let her change my last name to the new last name (no adoption, just a name change). I asked him about that recently (I am his only biological child) and he said it was more important to him for me to feel like I wasn't left out than it was to have me walking around having his last name. I can't say I felt one way or the other about it since I was so young, but it was nice being the same as my mom and my sister. (granted, I'm 35 so things were slightly more traditional than they are now as far as blended families, hyphens, etc) But it would have been just as nice being the same as my dad. I think if you explain what is going on and listen to her side, she's old enough to 'get it' and have some input maybe but for sure some understanding. And she's beyond giving what my first response was "but I just learned to write my old name!!!"