Even though adults have pretty much co-opted Halloween these days, it originally was more of a kid thing, and still is in a lot of ways. Parents will want to preserve those memories, if for nothing else as blackmail when they get older.
Move The Flash
For the best results you’ll want to use an external flash, preferably one capable of communicating with your camera’s e-TTL system. Those don’t have to be expensive, you can find a reasonably good external flash for most Canon and Nikon models for around $80 and a sync cord that also works with e-TTL for about $20. For less than $100 you can have an external flash you can move around to different angles. If you want to spend a bit more you can get a wireless flash controller.
Having a flash you can move around will give you the option to hold it below the subject, which is called “ghoul lighting” for a reason, or off to the side for a more dramatic effect.
Get In Close
The biggest mistake most photographers make is being too far from the subject. Move in closer, frame the shot so tight that you start losing part of your subject around the edges.
Better to be too close than back so far your pictures have a lot of distracting background.
Break Up The Police Line
Most people group everyone together in a pose reminiscent of a police lineup. Here are some tips from a previous article to break up the line.
It also gives you the chance to let the kids do something fun and be more expressive.
If They’re Wearing a Mask, Take Two Photos
If any of your kids are wearing a mask, take one picture with the mask on and one with it off. That way you’ll be able to identify the kid behind the mask years from now.