To keep it simple, I would SITD all the red/pink borders. You really don't need to quilt those borders since they are so narrow (you could do ribbon candy in the pink border if you think it needs it, but I really think it'll be fine on its own). Then, I would outline Elsa & Anna, the trees, their dresses/cape. After that, you could definitely do an all over stipple or crosshatch (either would look nice) in the remaining areas. It will take enough time just to do that & really, the only thing the little girl will focus on when she gets it is that she got a quilt with her favorite Disney characters.
If you're doing more than one of these quilts, to differentiate, you could always just quilt straight lines every 1/2" in the piano key border & **Astrix** in the pink border (call them snowflakes), but that's totally not necessary. It's a cute quilt as is & the most important thing is to get 'er done!
Good luck with all that quilting!