The only way the mother wren will accept the chicklets back into the nest is going to require some doctoring up on your part. Wrens are very intelligent birds. You will have to act accordingly. The best course of action is to set up the scene to make it look like a crime occurred, such as a group of drunken robins flying erratically into the nest on their way home from the pub. Drive to your local hardware store and purchase some bright yellow tape. Using latex gloves, carefully form the tape around the chick's bodies. For full effect, I recommend to fold one of the chicklet wingies underneath its body, so as to appear broken. As it stands, chances are it is probably injured or broken from the fall anyway.
Next, secure a pair of binoculars and hide in some bushes nearby. On average, most mother wrens usually notice a crime scene and act hastily. She may have an ex husband who is on nest incarceration and does not want to get his feathers ruffled. If all goes well, mama wren should swoop down and grab the chicklets and return them safely to the nest within an hour.
If, however, the crime scene has not been staged properly, the mother wren will just simply chalk the chicklets up as a loss. Typical female wren behavior after this includes, but is not limited to: Slapping on her best Covergirl makeup, fluffing out her chest feathers, and flying off to find the next available male to bring back to the nest to bang her brains out. Thus begins the evolutionary process in the life of a wren all over again. To the mother bird, it's a wren/wren situation.