Adultism is a term describing the arbitrary mistreatment of children and young people based purely on their age. The kinds of behavior that are often used to describe adultism would be considered socially acceptable in many societies. People who crusade against adultism suggest that children should be treated more like adults and given more of a say in their own futures. They acknowledge that adults have certain responsibilities for the well-being and education of children, but they don't think these should be exercised arbitrarily or used to justify repressive behavior.
Activists against adultism don't argue against parental authority over children. They still think parents have a job to tell children what's acceptable, but in many cases, they have a problem with the tone people use when disciplining children. For example, people tend to yell at their children or embarrass them in front of other people while disciplining them. Many activists would generally argue that this sort of behavior is dehumanizing for the child and that it has the potential to make the child into a more malleable and conformist adult.
Another aspect of adultism is the arbitrary dismissal of ideas ands opinions that young people may have. Young people often have a lot of ideas about what they want from their lives, and some people will totally ignore these because they feel that adults always know better. Those who disagree with adultism suggest that it's better for people to give children more say in their futures and listen more closely to their opinions.
One of the ideas that has grown out of the anti-adultism movement is unschooling. This is when parents allow children to choose what to learn without forcing them through a particular kind of educational program. Children are generally given access to whatever resources they need so that they can learn the things they're most interested in. Different people approach unschooling with different levels of structure, and those who favor this educational approach suggest that parents have to adjust it to the learning style of their particular children.
Many people disagree with the idea that adultism is anything more than the natural instincts of parents. They would generally suggest that people have a innate desire to make decisions for children because they instinctively know that they aren't mature enough to be treated as equals. Some even suggest that people need to be applying more discipline and setting more boundaries for children instead of creating a more loosely regulated environment.