Skip the box joints and consider some type of joinery which offers mechanical resistance in at least one direction. For example, mortise and tenon or hand-cut dovetails resist movement along one axis. The amount of effort you'll expend using a jigsaw and cleaning up the cuts with a chisel will almost certainly exceed the effort required to join the corners with hand-cut dovetails. With the box joints, you won't gain the inherent mechanical resistance; and if you have a cheap jigsaw (<$100), your cuts will require a good amount of cleanup, regardless of the blade you use, simply because the guides on cheaper jigsaws allow the blade to deflect more easily.
Also keep in mind that, as a sandbox, the corners will have to resist a tremendous amount of pressure and movement as the humidity changes day-to-day.
Be careful with your selection of glue, as PVA glue can "creep" or allow movement under constant tension and/or with high temperatures. Polyurethane glue hardens during curing and does not melt or creep once cured.