No, she is not too young. I learned the basic process when I was 6 years old from a friends brother. Kids hear things all the time. It is our responsibility as parents to let our children know what is right and what is wrong. If that mother is thinking that her child doesn't know anything...
The Ratio to replace 1 egg in a recipe is as follows:
1 egg- 1/4 cup applesauce PLUS 1/2 teaspoon Baking Powder
2 egg- 1/2 cup applesauce Plus 1 teaspoon Baking Powder
Etc.
The baking powder is the key, it must be used with the applesauce.
I'd take him home or to the hotel and change us up, make sure he's ok, and then go to the reception if he feels up to it. Usually people feel better after they puke.