There is certainly room for improvement with this design but it worked fine. As shown in Figure 2, make a track using two 8' long 1.5" x 3/4" wooden studs (Home Depot) and attach them with three pieces of wood as shown so that they are 3/4" apart. The first two attachments are about 1.5 feet apart. Drill a large hole all the way through the middle brace as your swivel pin will go through here. Make a small car with a small piece of the same 1.5" x 3/4" wood (oriented broad-side facing the floor) and attach four wheels to it (Home Depot sells small plastic wheels in the sliding door section). Attach two small screw eyes to the bottom of the car. These should fit between the gap in the wood brace without touching but try to make it a close fit. This will help keep the car on the track.
Now attach two more screw-eyes to the car (one in front and one in back) and four screw-eyes on the track as shown. It is best if you paint the track black. Tie some black string to the car as shown.
Drive a bolt through the roof frame (the top frame of the room the spider is in) and place the hole in the track through the pin. The room should be small enough that the track can not fall down into the room. You can attach a support rope to the short end for safety (since this is above the kids, you don't want anything crashing down because something broke when the kid grabbed the basket of candy and yanked). You are now ready to attach the fishing line to the spider. You will get a better reach if you attach the line a couple of feet from the end of the leg rather than at the foot (as shown below). You are now ready to operate your spider. Pull down on the short end of the track to lift the other end a few inches off the roof frame. You can then pivot the track over to the kids. Pull on one string to extend the arm and the other string to retract it. Get ready to yell at the kids when they get grabby. I put just enough candy in the basket for one piece each (letting just a few kids in at a time) and filled it between batches of kids. Since this was under black light, I got candy with glow-in-the-dark wrappers so that the candy would be easier to see.