Regardless of if your costume is bought or homemade, please use a "homemade" face. As a volunteer with our local fire dept and a first responder, I find it sad that every year at least one child is injured by their costume.
Instead of a mask, consider using makeup to create the face. A mask is often bumped out of place and blocks vision. Some masks also contain lead (from the paints) which can be inhaled. Other times small children have had them block their nose and mouth while sleeping and have suffered from a lack of oxygen. So instead of a mask, draw those whiskers and such on the face.
Also, if your child's costume has a tail, have them practice stairs with it. You may need to shorten it so that they don't trip over their tail. Don't let them ride their bikes with sandals, tails, or other costume parts that can be tangled in the brakes or spokes of the bike.
Also avoid materials that are highly flammable. Try to make it with pajama fabric if you can. It is marked as being flame retardant. One step too close to a Jack O Lantern can result in third degree burns.
Try to place some reflective tape on all sides of the costume and give your child a flash light. Avoid costumes that are of dark or black colors.
Lastly, please don't use draw strings around the face or neck area. Use elastic string instead, and make sure it is loose and won't choke a child if caught on something.
Source: very sad experiences