The trait does help Sims learn musical instruments more quickly. I remember taking piano lessons when I was 4, so children should be able to play some instruments...would be nice.
That was my reasoning as well, real-life children often start learning instruments at a very young age.
As a toddler did you have your Sim play the xylophone? That will give them a boost when she/he starts as a teen...lvl 3. I don't believe there are any musical toddler books, just writing, painting and logic?
As a Toddler my sim maxed the xylophone, and then when she aged up to child I gave her the virtuoso trait. I was wandering whether or not to teach her with adult skill books (like the ones for gardening, logic, cooking etc), but now I realise there aren't any books like that, I was getting confused with the musical composition books.
I guess the virtuoso trait is a bit useless to her, until she ages up to teen.