When the two girls are playing the violin, both of their bows hairs are loose instead of tightened as they would be if actually playing. This may seem minor... but to a violin player this will stand out like a sore thumb, screaming "THEY'RE NOT REALLY PLAYING!"
While playing violins, both bows run up and down the strings. Even an intermediate player will have long learned to run the bow as perpendicular (90 degrees) to the strings as possible, the bow playing on a very small area of the strings. Its very clear the actors do not know how to play the violin-- like watching an actor "play" a guitar but their finger movements not matching the music.