I would probably tie-tack in several spots inside that 7" tall area. You could use contrasting, coordinating, or a blending color thread or yarn. I would probably do it by machine (you can use a decorative stitch) just because it would be difficult to pull yarn or heavy thread through all those layers. Or, you could machine quilt decorative lines through that area.
Next time you might want to use a polyester batting such as Hobbs PolyDown, which will not shrink. I have also seen minky baby quilts done without any batting at all.