Every Song in the World
The first time I got this question, I was completely taken aback as to how to answer, and probably tried to say something along the lines of "it would take an entire lifetime to collect every song in the world, let alone play all of them."
But a 7-year-old's world view is pretty small. The immensity of the world, and all its peoples and histories, and the sheer amount of music that all that results in just doesn't compute yet.
So, after some thought, my new answer is, "Not yet, but I can learn any piece of music in the world, and you can too." The reason I opt for this answer is because I like to remind my students that I am still learning new things too, and that there is not a point at which we know everything there is to know and get to sit back and quit learning. I also prefer this answer because it is an opportunity to remind them of their own potential. The world is a big place, and it is chock-full of music waiting to be played, and there's even more music out there still waiting to be written. So 'every song in the world' is not a limit we someday reach, but an infinite opportunity we can use to continue growing, learning and making music.