Costumes should be made of flame retardant material. If they are home made, make sure and use either a fire retardant spray or fabric that has been treated with fire retardant.
Make sure to work into your child's costume reflector strips, a flash light or light stick to make them more visible at night. This will assure that they are seen by drivers and that you can keep an eye on them as well if they get ahead of you while trick or treating.
Do not include high heel shoes or long dangling pieces of costume that your child could trip over. A scraped knee or bloody nose can really ruin their fun.
If they are carrying a prop such as a scythe or sword, make sure they are made out of a flexible plastic that will bend if fallen on. Most of the props available today are made of flexible plastic.
If your child is wearing a mask, make sure that the eye holes are big enough to see not only in front but peripherally as well. You may have to cut them open further yourself. If so, use some black or gray grease paint around the eye socket to cover skin.
If you and your child are going out with a group, make their costume a unique one so you can spot them easily in a crowded area. Use flashing lights or jewelry to make them stand out.
If you have more than one child, save costumes from year to year to pass down or to exchange with friends. You can add extra accessories or dress them up a little to make them different from year to year.