We always keep a box in our trunks and in our office with things for children - non-damaging, non-noisy things. Some children's books, some stuffed animals, little squeeze houses or balls. When you're going through trade shows, some of the giveaways may be useful things for your collection.
If you know parents will be bringing a young child to closing, bring some things to amuse them, just in case the parents haven't. Don't bring markers that will damage the table of the walls, but an etch-a-sketch might work. Don't bring little papers and glue - but bring a few stickers that easily peel off. Again, the items in the truck box will work also. And if you're able to amuse and/or hold the child, it really takes the pressure off the parents.
Sharon, we have a basket in the conference room of our office for the same exact reason. The parents appreciate being able to concentrate on the paperwork, the child is happily playing in the corner. I hadn't thought about putting one in my car! Great idea!