Thanks for all your requests for more information on making a dump truck from a cardboard box. I used one nappy/diaper box for the cab and frame and a second, smaller box for the dumping bed.
To enable the dumping bed to move you need to create a hinge along the back of the frame where the bed attaches to it. Here are three possibilities.
Firstly, a folded piece of cardboard. Place the folded edge along the back and attach the cardboard to the top of frame and the bottom of the dumping bed. A thicker piece of cardboard will give more support to the dumping bed, however it may also mean that the dumping bed is never completely flat when sitting on top of the frame. The overall look and size of the dump truck you are making may help you decide if this is the hinge you want to use.
Secondly, pipe cleaners. Make holes in the back of the frame and dumping bed and thread the pipe cleaners through. I have twisted the pipe cleaner ends together inside the dumping tray. For more support, use more pipe cleaners along the back edge. If you choose to use this type of hinge you may find it easier to make the holes and thread the pipe cleaners through while making the frame rather than waiting until it is finished.
Thirdly, a cylinder such as a piece of dowel, skewer or straw, depending on the size of dump truck you are making. You will need the sides near the back of the frame to be higher than the level at which the dumping bed sits. I added some extra cardboard but it may be easier to cut the frame with this shape when you are making it. Make holes in this higher part of the frame and also the sides near the back of the dumping tray and thread a piece of dowel through. You may wish to attach something larger than the holes you have made onto the ends of the dowel so that it does not slide out (or get pushed out by busy little fingers).
Please let me know if you have more questions or if anything could be clearer.