We are in the final stages of adopting kids ranging from ages 7-12. The question has come up a time or two about last names, namely if they should keep their last names, or if we should give them ours, or perhaps a hyphenated name. What I would like to know is, what are the pros and cons of these 3 options, and how much should we involve the kids in the decision?
Let each kid decide.
I changed my last name when I was ten and my mother remarried. I got to make the decision myself, and I don't think I did the wrong decision. And don't forget to consider the names themselves in the decision. Unusual names have their benefit as you get less mixups, and names should be easy to pronounce in many countries (ie no weird characters and few 'R's.)
But in regards to your question, the only drawback with having a different name from the rest of the family is confusion, people wonder why your kids have a different last name. I believe that was a big part of my older sisters decision to change, she initially kept the old name.
Other than that there is no particular drawback to having a different last name from your parents, and the only drawback with a hyphenated name is that it's long and confuses the people who make up usernames on company computer systems. :-) Although that could be seen as a benefit... Hyphenated names also sound fancy.
So I think you can let the kids make that decision themselves. At least assuming the new names doesn't end up embarrassing them. Both McDonald and Berger are fine last names, but McDonald-Berger is a bit unfortunate. :-)
It's also a good thing to let them feel they have control over their own lifes. And if they are unsure, they can always just keep their current names for the time being, and change later.