Bias binding ties that are a part of the neck binding have already been described. If you finish the front so the pieces overlap, you can put snaps or even small buttons in that overlap. But if you use any of the other options described, you still need a way to close up the top third of the front.
Ribbons are the easiest choice. Attach them to the back of the finished edges as described earlier for the second set of bias ties. Buttons can be added to one side and button loops to the other. The loops can be made out of stitched narrow double-fold bias tape, narrow ribbon, or yarn.
Snaps might be more practical. You can put the underside snap on seam binding. To do this, cut the binding to about 2″ and fold it in half the long way. Stitch across the end, and turn. Sew the stitched end to the underside of the gown so the folded end extends beyond it. Sew one half of the snap to the seam binding and the other half to the back of the other side of the gown.
Another possibility is to use little embroidered appliqués that you can buy in stores. Choose a pair about 1″ across. Stitch one edge to the edge of one side of the front so the center of the appliqué is along the center front edge. Sew one half of the snap to the underside of the appliqué and the other half to the other side of the gown.