Shane's right about the shops. We've been driving down from the Bay for the Open House for years, and it's the gentler of the two. You can wander at your own pace, and a lot of the stores put out cider or cookies, so the little ones usually end up with something sweet in their hands before you've gone a block. Go Friday evening if you can, when the lights are coming on. Saturday midday is nice too, just a bit busier.
One thing I'd add is to plan your supper ahead of time. Restaurants in Heber fill up on those weekends because everybody has the same idea once it gets cold and the kids get hungry. We either eat early, around 4:30 or 5, or we go before we start walking. Waiting an hour with grandkids who are already worn out is no fun for anybody.
Stacy, saving the parade until the youngest can stay up for the lighting is a good call. The parade will be there next year, and it's more fun when they can actually enjoy it. Hope the grandkids have a good visit.