These are very easy to make from recyclables.
Assemble your materials first:
You'll need straws, bamboo skewers, water bottle caps, and milk or juice cartons.
Tools you will need are a paper or box cutter (for adult use only!), scissors and tape and decorative paper, and glue.
The first step, putting holes in the bottle cap wheels, is for adults only, and if you have impatient little ones, it might be best for you to do this before making the trucks. In fact, if you prepare a whole bunch of these in advance you can set up a little vehicle factory. Your children could also get to work decorating their cartons while you do this part.
Take the box cutter (or a small knife) and make a small hole in the center of each bottle cap. I found that the easiest way to do this was to place the bottle cap on a folded washcloth on a table, place just the tip of the box cutter in the center, and press down while twisting the bottle cap around (by turning the washcloth, thus avoiding having to touch the bottle cap with your fingers and getting too close to that blade!). Definitely do put the bottle cap on some sort of surface to provide traction while cutting.
Next, cut your straws to size - just a bit bigger than the width of the carton.
Start assembling the wheels and axles. First, stick the end of a skewer into one of the holes. Add a dab of glue if you'd like.
You can actually stick the skewer in the bottle cap the other way which might be easier for kids because it's more stable and less frustrating.
Next, add the straws.
Once the straw is in place, trim the skewer down with scissors (another adult-only job), and stick the skewer through another bottle cap.
Now you can tape these onto the bottom of a carton and your vehicle is done, aside from decoration, which is what your children will have the most fun with.
If your children want to make a dump truck or cut out windows and/or doors, they should do that before you put the wheels on. Making a dump truck is very easy - you just cut out part of the side and bottom of the milk carton and then stick it back in the other way, attaching with tape. It really works! Fire engines were popular in our house too as were buses and passenger cars.
Once you have the wheels and axles down pat you can stick them onto anything, really, like water bottles or small boxes. If you want to make trains, connect the train cars with the bit of straw you cut off the axles (this is especially great if you use bendy straws).
Here are some photos from Japanese children's craft books for more ideas! I especially love the corrugated cardboard around the wheels in the last photo.