I would quilt each of the horses in a color that matches the fabric with one design & would quilt all the white in another design (in white thread). I'd do horseshoes in the sashing, possibly in a contrasting color.
Here are a few ideas for what you might consider for the quilting design (note: the bubbles would go all the way around the horse, it just was getting late):
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I'd also embroider on an eye & a mouth on each horse in a dark colored floss/perle cotton. You could do that on the machine with embroidery/thread painting or could even hand stitch a simple French knot for the eye & a running stitch for the mouth. For some reason children seem much more enamored with 2-D animals when they have a "face".