Spread a geotextile sheet over the entire excavated area.
Nail together the four 2 x 10 pieces that will edge the sandbox and place them inside the perimeter (over the geotextile sheet) and resting against the posts at the four corners, without extending beyond the borders. According to this plan, your sandbox will be 9 1/2 ft. x 7 ft. The border will be buried 4 in. into the ground. Backfill the outside.
Attach the four corners of the border to the four posts with lag screws.

Cut six 4 x 4 posts into 7 ft. lengths. Using a bench saw, make a notch 1 1/2 in. deep and 9 1/2 in. long at the bottom of the six posts.
Set the height of your buzz saw blade at 1/2 in. before making a notch into the six posts, at 2 ft. 7 in. from the unnotched end. On the four posts already notched, this second notch should be on one of the lateral sides in relation to the previous notch so that the two notches are on adjacent sides.
To build the platform beams, cut four 2 x 6 pieces into 5 ft. 5 in. lengths. In your plywood, cut out two pieces 5 1/2 in. wide by 5 ft. 5 in. long.

Make two beams by inserting a plywood piece between two 2 x 6 pieces. Glue and bolt the three pieces together.
Insert each beam into the lateral notches in the two corner posts and, using an electric drill, screw them in opposite the notches from outside the posts.
Place the notches at the bottom of the corner posts on the sandbox border and centre the frame between the structure posts (peaks). Bolt to the structure and to the sandbox border.

Place metal hangers for the 2 x 6 floor joists on the beams so that once installed, they will be centered 12 in. apart.
Cut joists out of 2 x 6 pieces. You will need a total of seven 9 ft. long joists. Also plan two other 9 ft. 7 in. long joists for the ends.
Attach the joists to the hangers and double the border joists by attaching the two pieces that make up the ends of the posts to the outside edges of the border joists.
To make the studs, cut three 4 x 4 posts into 3 ft. 1/2 in. lengths. At one end of each, make a notch 2 in. deep and 5 1/2 in. long.

Position the two remaining 4 x 4 posts, placing the notches out, over the border joist before attaching them to the joists. The first post will be attached to the joist 26 in. from the outside end of the 6 x 6 post and the second 18 1/2 in. from the first measurement.
Use the same procedure to attach the three studs. Position the first stud 18 1/2 in. from the last stud attached in the preceding step, and the other two at the opposite end of the platform, each 26 in. from the outer edge of the 6 x 6 structure, on the border joist(notch out).