We send children to school without their consent.

This can be argued to be morally unethical, although some people don't see it that way, but that has more to do with how people view children both as people and property.

In fact, we force parents to send children to school without their consent.

We compel education. We don't mandate sending children to school.

I think to be more accurate, you mean to say that since children can't consent, who will consent on their behalf? A freely elected government generally does, since the state protects children in the same way it protects the mentally handicapped.

States don't just define a minimum age of consent. They consent on behalf of children. In that sense, we can compel them to go to school "without their consent," so to speak, because the state is the only institution that can really legitimately claim to consent on their behalf.

The state has stronger claims over children in some cases than their parents do.

Children of any age are legally able to work on farms owned or operated by their parents, which would disagree with your premise. If parents are able to legally assume consent for their children to work on the farm, they would be able to consent for all employment opportunities.

For what it is worth, working on the farm as a youngster brings some of my fondest memories of childhood. I find it rather sad that people outside of the agriculture communities do not have similar opportunities until they grow older.