Feel free if you want to explain how slavery is bad. That topic is highly biased anyway. Don't waste your time for pointless topic. I would ignore your point for this anyway
Back to my previous point to explain it further. Employment and slavery have different impact in this world that have tags for many people in it.
An 'employee' means he can stand by himself without any need to have guardian for him. A 'slave' means he is owned as property by the owner and the owner (if its responsible) need to properly give his 'property' any kind of support to make the slave live longer.
And if it's a 'kid', this would give them a more advantage (in the same kind of responsible owner/employer). The reason is :
1.
This is the general idea of people that lives in this world
2. A kid employee is easy target to be kidnapped again by more worse slave trader. If it has the job 'slave' by strong company, other people would hesitate to do bad things to the kids because they are afraid if the kids actually someone 'favorite'. A kid employee would be looked down upon with pity and remorse by society too. That's nothing different with being a real slave. The job 'slave' actually protected them from being eaten by sefina in the plot story.
3. Myriad's intention is to make them move to proper place, not raising them. It would make it more attractive to the buyer with the fact the kid is a 'slave' than 'employee'. Who would expect the kid 'employee' to have an actual skill in the job. And the rights they hold would give more weight to the potential new owner.
4. Lifting the slave job is easy you just need the owner's agreement to do that.