My opinion is that part time jobs at an early age teach things like time keeping, hierarchy, customer service, teamwork etc which are all important skills to have
I remember my first job when I was 16 at the co op. One thing I quickly grasped was that even though I was young I wouldn't be treated like I was- I was there to work like an adult
But equally you could get some of that elsewhere- sports clubs, voluntary work for example
I believe in let them have a wall to wall childhood, other than school work. Plus kids do get tired. From age 11 I had a stack of homework to do due to the sets I was in, in maths alone hours a week. If that’s done with effort, that‘s good enough for me.