We started decorating our side yard and garage about four years ago, expanding from just the front yard set-up. It has developed into a scary part (the drive/garage), and the kid friendly part (front yard.)
We put a barrier up across the drive and let people know that the driveway walk is meant to be more intense. This year, we're erecting a 12 foot by 8 foot castle facade. We have costumed helpers who instruct those with small kids not to go through if they might be frightened. We also give candy out in both locations. In the past, we've had a few "double dippers" but not many. Crowd control was an issue last year and it's tough to keep track of who might have gone through, but IME, real little tots will be content to get their treat in the front yard part, while the older kids want to do the walk through. We also have helpers in the front part of the yard to hand out candy to the youngest kids. Your going to get a few who try to hit you up twice. Having a few assistants is a big help. We also use portable walkie-talkies to communicate potential problems between the two areas. The biggest problem we had was running out of candy in one area and then having to send a "runner" to get some from the other part. It caused a fw back ups. Just be sure to have enough candy on hand so it runs smooth.