In most instances I would say no, children are not oppressed by adults. Most caring parents encourage their children to grow and flourish instead of oppressing them. Also, when children are at school, I believe that most teachers and administrators have the child's best interest in mind or they would have never gotten into that field of work in the first place.
Children are not oppressed by adults in general because the children have to grow up first. It's not oppression, but a natural progression from young children to adulthood. Oppression may happen, but only in cases of abuse. Adults who oppress children are not their parents, and it is the responsibility of the parents to prevent this oppression at the hands of other adults.