I definitely agree with starting them out on a snap kit, but make sure they're the ones that choose the models. Also, it may be extremely difficult to hold back, but never force them to add details that they aren't concerned with. If they fall in love with models, they'll start to garner an appreciation for fine details and craftsmanship, but right now, they're likely interested in taking a bunch of separate parts, and turning it into a toy car. They WILL want to play with the car more than stare at it on a shelf. For that reason, the Mustang would be an awesome start.
When I was a kid, my dad was working on a ship model, and very focused on the details, for that reason, I completely lost interest in ships. When he just let me go on my own to snap together my car models, I was thrilled. My models were finished the same day I started, and I didn't care.