I agree with the idea of a meander in the pink & white areas and SITD around the printed blocks and a straight line stitch tracing the printed border on the prayer blocks. I did a quilt recently that was a bit similar, but it had applique dresses as the featured item. The dressed had ruffles & rick-rack & iron-on buttons and all sorts of little details that precluded everything larger than about an 1/8" design (which I'm not skilled enough to do), so I just left those areas be. It has been washed at least a half dozen times already & I've yet to see any sign of a problem from the open areas.
It's a sweet little quilt and I agree that quilting over the little girls' faces -- even with mono thread -- would probably distort them. You could maybe quilt a large inner circle in those blocks just to add a bit of detail, but I wouldn't risk more than that.