You do realize that your wife most certainly CAN use the iPad in the car to do offline things, like reading iBook content or other already downloaded material. I bet if you talk to her about what she would like to do with it, she very well may be able to use it exactly like she wants in the car, waiting on the kids.
The iPad has plenty of functionality when not connected to the net. Heck, I use my iPhone on flights all the time to watch movies and shows, listen to music and play games.