That is a question to those, who is interested in etymology. In the article, there is explained why working children is not a bad thing and helping your parents is much smaller/no problem (nowadays), because everyone approves it.
But helping on your parents farm is definitely* not slavery, because you always have the choice. Sometimes you choose between helping and worse relations; sometimes between helping and leaving your family (and probably your friends, school, other interests, ...) and sometimes you don't know what the second choice is.
*it is slavery only if they violently forces you to work and to stay in the family and you can't leave even without telling them.