There's a matter of degree as well.
Expecting kids to help out around the home, or pitching in at the family business is reasonable - so is expecting older siblings to help with the younger ones.
Expecting a child to work two full time jobs, I think, is not, and that's what it comes to if they are attending school full time, and spending all their out of school hours working at a family business.
In the past, kids working on the farm or in the home business was a normal part of life. It was also normal for kids to be kept home from school during the harvest or to leave school at age 15 or younger to work, or to leave school if they were needed to help at home. Going to school was often seen as secondary to supporting the family, unless the family was well to do - my one grandfather had to quit school to work because the family needed the money, the other was raised on a farm.
Of course, the kids were also being trained in a trade that would subsequently earn them a living, even with minimal schooling (or they were girls, and weren't expected to earn a living in the first place). This is no longer true - practical experience in carpentry or running a business won't get a young adult far without at least a high school diploma, and generally post-secondary training as well.
In modern society, attending school is considered a child's primary job. It is illegal to take your child out of school to help support the family, or to have them look after younger siblings full time, and children are expected to attend school, and do homework, unless ill - it's busy season at the family business is not an accepted excuse.
In the particular case here - the grandson was being used for hard labour without his parents' permission or knowledge. He wasn't being paid, by the sounds of it.
And the level of what he was being asked to do may or may not have been appropriate for his age. The OP said "tween" - so probably about 12 or 13 years old, likely with no particular training in construction. He's being asked to do roof work, heavy construction, electrical and plumbing work, which is pretty intense and potentially physical dangerous work, particularly for someone with no training.