Cut the PVC according to the height of your children. You will build three frames, each with a cross brace about 2/3 of the way down. One larger frame will anchor the face of the theater, and two smaller frames will form the "wings." You'll need six longer upright pieces and six shorter ones, with three 34" cross pieces for the front and six 16" cross pieces for the "wings." You'll also need about 4' of thin PVC (it has to be able to slip over the frame pipe) in the next largest diameter to make the hinges. For a 3' to 4' tall child, make the longer uprights 36" and the shorter ones 24" long. This will total about 48 running feet of PVC. Be sure to cut the pipe squarely (use a miter box or pipe cutter if you have one). Smooth the rough pipe ends with an emery cloth and wipe clean with a soft cloth.
Since the bond created by PVC adhesive is very fast and tight, pre-assemble or "dry fit" all parts before beginning. With "T" joints, join one short and one long pipe for each upright, and put elbow joints at both ends of each. Join the uprights together with the cross pieces, three to each frame, using the 34" pieces for the front and the 16" pieces for the two "wings." Check for fit and squareness. Make any corrections now, while it's easy to take the frames apart.
Make four "hinges" by cutting eight 6" pieces of PVC in a larger diameter (use the thin-sided type) that will slide easily over your frame pipe. Glue two pieces side by side so you have four double pipes, each 6" long. Allow to dry thoroughly.
Reassemble and glue the six uprights of the frame with long and short uprights, joined with the "T" joints. Do not glue elbows at either end. Slip hinges on each of two uprights to make two sets of two uprights joined by two hinges, one below the "T" and one above.
Glue elbows to the bottom and top cross braces, taking care that they are squarely attached. Working on a flat surface, attach the rear wing uprights to the hinged ones by gluing the (16") center cross pieces into the "T" joints. Glue the bottom and top cross braces to the wings. Be sure the frames lie flat.
Glue the center (34") cross brace between the center "T" joints. Again, working on a flat surface, attach the top and bottom cross brace elbow joints. When you set your frames up, you'll have a three-sided box. To keep your hinges from "floating" or sliding around and shifting, attach them to the front frame (not to "T" or elbow joints) with short wood screws so that they are perpendicular. Your wings will still move, but they will be more stable.
Cover your frame with material to conceal the puppeteers. If you're using plywood or composition board, cut a proscenium (stage opening on the front for the puppets) and attach them to elbow and "T" joints with wood screws. If you're using corrugated board (refrigerator and washer-dryer boxes are perfect) or canvas, attach with hot glue and cut your proscenium after attaching it. You can also cover your pipes with canvas using grommets and lashing if you're handy with rigging. Leave the center cross brace free from glue so you can hang puppets and props.
Finish the theater at the direction of your little puppeteer--you've been in charge long enough. Tie a rope between the wings at the top so they are not accidentally kicked open (and to use to secure the wings of your theater when it's folded for storage). Decorate your theater with tempera paint, wallpaper, contact paper or faux finishes with fancy lettering. String a curtain along the top inside. Children can stand behind the stage to use the theater for hand puppets or they can turn it over to use for marionettes. Just string a drape between the wing cross braces to mask the puppeteers' feet.