Laura's dress was made from this pattern (McCall's M4547):
Mary's dress was made from McCall's M9424:
I would have used this last one for both, but it wasn't available in "peanut" size. If you want to take on a project like this, there are a variety of patterns to choose from the various companies. Between the two of these, I would recommend the second one. It was easier to follow and personally I like the way it turned out better.
Think we're going to have to go back and reread the Little House series again this winter!