Tolerance, one of the core values of the child liberation movement, means showing deference to others' rights to engage thoughts and behavior one may personally oppose, but which do not infringe anyone's rights. The child liberation movement favors people's manifesting tolerance toward everything except intolerance itself. There is a difference between holding and advocating a belief that children should not be freed, and using force to prevent children from being freed.
The age discrimination currently practiced against children is often based on intolerance for behavior that people would like to see criminalized across society. It is easier to pass a law that only criminalizes behavior when it involves children, because children are a politically weak group. The prohibition of children's participation in an activity can be a "wedge" by which one harms and stigmatizes whatever industry or community is involved in the behavior and paves the way for abolishing that activity. For example, producers of pornography, alcoholic beverages, and the like suffer loss of revenue from children's not being allowed to buy their products, and nudist families are inconvenienced by restrictions placed on children's involvement in nudist communities. For the parents to participate in activities from which children are barred would require finding alternative child care, which can be expensive.
When children are barred from an activity, it often implies that it is a dangerous or unwholesome activity that perhaps adults should not engage in either. They can be deemed to be setting a bad example by doing so. E.g., it is deemed to be particularly bad parenting to smoke tobacco within sight of minors; once the child has reached an age at which he could legally smoke it if he wants to, he is also deemed to be old enough to be exposed to others' smoking it without being influenced in the same way that a younger person would.