Thanks for all your answers. She had never experienced lower arm soreness before till recently when she started playing long pieces that require her to reach octaves at many parts. She can barely reach 1 octave yet. She is not tense when she plays the songs, she enjoys playing them but she’ll try to stretch her palms bet thumb n little finger quite laboriously. She plays the octaves well if the notes are separated but if the octave notes are to be played harmoniously, she will have to stretch, resulting in the soreness. Could it be her hands have not grown in for playing certain songs, got to wait for her fingers to grow longer?