You're not their mother, and you never will be. They're conflicted enough, and pushing them to use a mom-name will only confuse them more.
Corollary: "We're going to be one big, happy family!"
You might eventually become the happiest of stepfamilies, but it won't happen overnight. Studies show the new family dynamic takes at least three years to fall into place, and the first year is the toughest.