I'm not sure - except have some way to let your co-workers "opt out" if they want to. Halloween is my favorite holiday, and we celebrate to the nines...but at home, not at work. I personally wouldn't want to have kids stopping by my office on halloween - too distracting for getting stuff done. That's what halloween night is for - the trick-or-treating fun. :-)

I'm usually just a lurker here but I thought I'd let you know that I have some experience with this. Last year was my first Halloween at this job and we do office trick-or-treating here. It's on a volunteer sign-up basis.
You can opt-out, donate just candy, donate candy and be a trick-or-treat stop, and you can bring your kids into the office from 3-5pm. Basically our department shuts down between 3 and 5pm. Some people go all out and decorate their cubes and even dress up. The only rule is nothing too scary since there are a lot of very little kids.

I signed up to donate candy and be a 'stop'. My kids are older so they won't be coming in. Our department secretary coordinated the volunteers and she had balloons placed in the trick-or-treating cubes so that the kids could see where they could stop for treats. They just had to look up and if there were balloons, they new exactly what to do!

I brought in 2 jumbo mixed bags of candy and I stopped at the dollar store and picked up some trinkets - plastic rings, Halloween pencils, stuff like that. I decorated my cube but didn't dress up. The biggest hit was the plastic rings that I grabbed on a whim! Who would have guessed??

It was the best time! So many people were new here last year and didn't participate, and wished they did. I heard that there is close to 70% employee participation this year and we're talking about close to 100 employees - similar to your group!

Someone set up a conference room with decorations and crayons and the kids could go there and hang out to sort their candy. Plus the adults took a break from work. I think we also had drinks and paper goods from the cafe in there, but i don't remember exactly.

It really was a great time and I can't wait to do it again this year. Our office is doing it this Friday and we are all starting to put up our decorations tomorrow.

Good luck!