When my first kid was small enought that I could steer him into the costume I wanted him to have, we'd dress up to match. We were both dogs when he was 2, bats at 3, a jaguar and zoo keeper at 4, then 5 threw me off. At 5 he wanted to be a Power Ranger and I wasn't in good enough shape to be a villian (there was a cool villian lady at that time who wore lots of black leather) and I sure wasn't going to be a giant Pink Ranger. So I was a Mummy. Worst costume I ever made - I dressed in white long underwear and wrapped myself in strips of cheese cloth...that kept falling off. Halfway through trick or treating my butt was hanging out...Good thing it was dark. After that night I just wear my witch hat and a black cloak. I also helps that we changed neighborhoods and now he has enough friends nearby to trick or treat with...
This year my oldest is going to be Dr. House (he's 10). It's a weird costume for a 5th grader, but we do watch a lot of House, so I guess it can be expected. I thought about dressing as a patient, but then changed my mind. Little brother is old enough to go trick or treating too this year, and he's going to be a lamb (to match his lovey).