Thanks, everyone, for your replies. So, we went through with the experiment. For some reason they insisted on Denny's which was probably good, because none of it tasted very good to them. My 7 year old (who was the most adamant about this) only ate 1/4 of the burger and the next day said, "It actually didn't taste so good." My 9 year old ate more of her burger and was very constipated the next day. Both had the dairy desserts, but again, didn't eat alot, but I think they ate enough because they noticed runny noses over the next few days. They said they like soy delicious better than the dairy ice-cream and that is something we can easily do once in a while.
Overall, I think it was a success, because what they were really after was knowing "what they were missing" which now they don't think is very much. So, we're currently in the kitchen making baked falafels and hummus which they love. So, back to normal!
Jen, happy to hear your experiment went well.
Baked falafels? Is this recipe from McDougall and available online? If not, would you mind posting the recipe, please? I'd love to try these.