Much as I hate mail-order costumes with all their clichés, cheap fabric and shoddy construction, I feel ambivalent about the homemade variety.
I have bad memories of days spent slaving over some darling little thing only to be loudly rejected by a fickle toddler who declares ten minutes before trick or treating that he doesn't WANT to be a lion anymore.
A few years later I remember walking through the streets with a knight in shining armor who was anything BUT chivalrous: he wanted the same Ninja costume all his friends were wearing.
My kids and I have reached a compromise of sorts: either they pay for store-bought costumes out of their allowance (including tax and shipping -- which means they have to save for about 3 months for the average Ricky outfit with plastic accessories), or they have to graciously accept my free services, submit to fittings, and swear an oath not to change their minds at the last minute.
For my part I have abandoned the labor intensive full body outfits. Regular clothes with a few choice accessories can have just as stunning an effect. I don't want to spend more than a day on a costume.
These are not instructions on making any single costume, rather tips and photos which are designed to help you imagine and craft your own.

Step 1: Comfort

Comfort is the most important design consideration when making costumes for children. Not only is body movement crucial, but temperature control is too -- it doesn't matter how cool the Roman toga looks if it has to be covered by a winter coat. And no matter how frightening the mask is, it won't last a minute if it's uncomfortable and hot.

Where I live Halloween is a particularly difficult time of year to design for, because it can be crisp and beautiful, or freezing. Layers which are part of the design (such as a cape, or removable gloves and hat) are the best way to deal with the weather.

Allowing for body movement is very important, especially for determined trick or treaters. Any accessory must leave at least one hand free. If you saddle your young cowboy with a hobby horse you will end up carrying it around the whole time. Take all the pictures you want before you set off, then leave the cumbersome accessories at home.

To avoid loud complaining or an outright refusal to set feet into your creation, any cardboard based costume should no go lower than the hips. For a long skinny costume, such as the classic giant pencil, try making the lower part out of fabric (if it's attached at the bottom to a small hoop it will keep a well defined circular shape). Not only would it be easier to walk in, but the kid would be able to sit down and rest once in a while.

Step 2: Transform What You've Got

My sister, who is a much better seamstress than I, and much more willing to spend an obscene amount of time making costumes, had made a beautiful white kitty cat suit for my niece. The next year my boy would not hear of being a cute white cat, but he did like the idea of being "the sugar monster." All it took was a blue cape trimmed with (empty) sugar packs, a few other sweet (literally) decorations, and my son became a very enthusiastic sugar monster. Nobody got it, of course, the idea was a bit too abstract, but he still got lots of candy.

Step 3: A Few Key Accessories

Quick and easy costumes are essentially a combination of accessories. Any single element does not create the costume, but put together they gel. Walking around in an over-sized white shirt won't make anybody look like a doctor, but add a stethoscope, perhaps tape E.M.S. on the back and the kid will feel perfectly entitled to ask his or her friends to strip...

If you base elements of the costume on real clothes you can continue to use them on any other occasion. My son used the sweater and hat from his tiger costume for more than a year after Halloween -- on their own they didn't look like a costume, just cute clothes. With the King costume I reclaimed my velvet shirt on November first but my son kept acting like a tyrant till I took his crown (and the candy) away.

Step 4: Family Themes

By the time I'm done with my kids' costumes I don't generally have the time or inclination to do anything for myself (the last time I dressed up I was a fat dwarf accompanying my niece Snow White -- I was pregnant at the time). But even just with two boys themes work well. The sibling becomes an accessory. Each costume enhances the other.

Step 5: A Few Techniques

Although this instructable is not for any single costume, some elements do come up frequently:

Tails:
Make these with a long, rectangular piece of fabric, sew on the reverse (inside out) then flip it right side out and stuff it with pillow stuffing (this is the hardest part). I also use wire (the solid core electrical wire is perfect: not too stiff but it holds its shape) both to give the tail a good shape and as a means to attach it to the body. Use a wire a little more than double the length the tail, fold it in two and push the folded end into the tail before you stuff it. This way you can either pin it or sew it to the fabric at the tip of the tail so the wire won't slip out.

Full head masks:
Besides the simple cardboard tube style mask (like the one I made for the Knight's armor) the best way to make a mask is to build it around a helmet. Bike helmets sometimes have funky shapes, so a rounder ski helmet works better if you've got one. If you use a properly adjusted helmet you know it will fit snuggly and comfortably, and you can easily put it on and take it off. For Clank's head I taped and stapled some cardboard around the helmet so I could get the right curved shape and cover the front of the face. I covered that with tin foil, then wrapped the whole thing up with clear packing tape.

Gluing tin foil:
This classic of home made costumes material should not be glued onto cardboard with a few drops of all purpose glue -- it will be sure to rip. The only way to glue it is to use a paste which is painted onto the whole surface. Then if you reinforce vulnerable corners and edges with gray duct tape the costume will definitely last long enough for an afternoon (and evening) of trick or treating. Here is the recipe I used for my glue:

1 pack unflavored gelatin
3 1/2 tsp water
2 tsp skim milk

In a small cup pour gelatin over cold water to soften.

Boil milk (preferably in a pan of water) then mix it into the wet gelatin and stir till all the lumps have dissolved.

Adhesion works best when this is applied hot. If it is too runny for the surface you are gluing, wait for it to cool down and gel before you paint it on.

This can be stored in a small covered jar for a few days. Warm in a pan of hot water before use.

************************

Good luck!