Give it back and leave it given
The job's been theirs for a fortnight and it's roughly working. This chapter is about the two things that happen next, which are adding another job and not taking the first one back.
Add the next one when the first one's boring. Not when it's perfect. When the bag is being packed without you thinking about it, and without them making a thing of it, that's the moment. Pick the next annoying job and do chapter 19 again. It'll be faster, because you've both done this before and the child knows what "it's yours now" means. Over a school year that's perhaps a dozen jobs, and a dozen jobs is a different childhood.
Don't take it back on a bad morning. This is where most handed-over jobs go, and they go quietly. There's a tight Thursday and you do the bag, and on Friday you do it because you did it on Thursday, and by the next week it's yours again and nobody announced it. If you have to do the takeover on a tight morning, say so. "I'm doing your bag today because we're late. It's still yours tomorrow." The saying is what stops the job sliding back. Without it, the child correctly concludes that the job is theirs when convenient, which is the same as not theirs.
Don't take it back when they do it badly. They'll do it badly. That was the deal in the title. A week in, the bag will have the wrong book in it and you'll get a note from school, and the note will feel like a verdict on the method. It isn't. It's a nine-year-old learning what happens when the wrong book goes in, which is a note, and the note is a better teacher than you are because it doesn't love him. Let it land. Then ask him what he'll do about it. Don't put the right book in.
Don't take it back when they ask you to. "Can you just do it" will come, especially in the first week, and especially from the older ones, who've had the longest to learn that asking works. The answer is the one from chapter 22. "What's the first bit you're stuck on." It's kinder than it sounds, because it's an offer of the least help, and it's firmer than it sounds, because it doesn't move the job.
Notice what you've got back. This is the bit parents skip and I'd not skip it. After a term of this, the morning has fewer jobs in it for you. Count them, the way you counted the takeovers in chapter 1. The number will have moved, and the minutes will have moved with it, and quarter to eight will have less of the sigh in it. That's the return on the fortnight. It's also, I'd say, the sight of a household with more than one working member in it, which is what a household with children in it is supposed to become.