I like the idea of the yellow but is it a little too bright for the other yellows in the quilt? I'd hard to till from the photo. If you have a yellow that is slightly more gold than sunshine yellow? Otherwise, I like the suggestion of using the plaid that's used in the other blocks in the same rows.
The above was difficult for me to say, since I think all colors for children should be bright...but since other colors are slightly grayed, the grayer look would fit in better here, I think.