If you are advertising for diapers do you really want an adult to model it? If you answered yes to this question then you need to find a therapist. Kids are people to and don't always need to be treated worse than an adult. As long as child labor laws are enforced, it is perfectly fine. WAKE UP AMERICA!
Children can be used in advertising as much as advertisers want to, as long as child labor laws are enforced and as long as the child's parents approve. It is up to consumers to decide whether or not to believe the advertising. It's not like children will be smoking cigarettes or drinking alcohol in the ads.
Children are a huge part of real life so it only makes sense they should be present in advertising. It is wrong to force a child to be in a commercial, but if they want to, I think it's okay. Parents just need to be careful about children getting overworked.