Ukulele by Heather Ross, is perfect, both for the bag and for Lucia. One of her favorite things is playing the guitar with her dad. So awhile back he bought her a real, nice ukulele. I thought it was a little much for a two year old, but she has not only learned how to strum, she has also learned respect for the instrument (most of the time) and tune it (haha).
Back to the bag. The pattern is the toddler backpack by Made by Rae using the backpack instructions from the Oliver + S Little Things to Sew book. I bought the pattern before she added the lining instructions, so with the new pattern that should be all you need.