Great ideas for memorizing prefixes above so I won't add my 2 cents (we made up our own).
While prefixes are important, don't forget the rest of the metric system.
Start with about 20 drops of water. I have a set of tiny (a little smaller than a die) cubes that have one side missing. They each hold 1 milliliter (ml) of water which happens to be the same as 1 gram and the same as 1 cubic centimeter. If you have (or can get) similar cubes that hold 1 ml of water, it's a good hands-on visual.
One thing you can buy that are nice to use while teaching metric is:
It holds 1 liter of water and has graduated markings up the side. So it holds 1000 times what a tiny cube holds and you can actually picture those little cubes stacked up, 100 at a time, 10 high, in the cube.
Have your kids measure and estimate things. They are probably familiar with a 2 liter pop. They might want to get used to the masses of things. My scientific calculator is about 110 grams. A typical red brick is 2 kilograms. Remember that weight is not the same as mass. You'll teach mass first (grams is the SI unit) and only if you want to go deeper do you teach weight (newtons is the SI unit).
Have them learn distances. Put a dot on the paper and have them guess a dot that is 20 cm away. Then measure it with a ruler.
You can get a little pi lesson in too. Many kids can pretty easily estimate the distance across a circle (diameter) but few can estimate the distance around the same circle (circumference). Using round items (yogurt tub lids, coffee can lids, shapes you cut out), have kids guess how far it is around the circle. Then, they can wrap string or yard around the circle and then measure that string or yarn. Usually kids estimate low with circumference.
Have them see how tall they are. Guess first, then measure. And, know where a meter hits each child. Put a meter stick (a very nice one can be purchased on Amazon for about $7) up near each child so he or she can see where a meter is (probably somewhere on the chest). You can teach children to put their arm up to a meter high.
Not sure if they are ready to do area (a sheet of printer paper is about 600 cm2) or volume (a Nature Valley granola bar box is about 1 liter, and that liter cube is, of course, a liter).
Learn temperature too. A cool room is about 20 degrees, a stuffy one 25 and 22 is pretty comfortable.
If your special ed kids are ready they can learn volumes of spheres and pyramids in metric or, more simply, the area of a circle. A pizza is about 100 cc. Oh, and it's always fun to estimate the mass of foods. An apple is about 200 grams, a banana about 170 grams.
That should keep you busy for a while.