And see, in my personal opinion, I go beyond "when you can't afford them". I say when you are unable to care for your child yourself and must depend on someone else to look after them, then don't have anymore. It's not fair to the parents, the existing children, or the children to come.
My oldest child is medically fragile and has special needs, but we had two typical children after her.
In my opinion, there is a line between kids helping out and kids raising kids.
Anyone is allowed to have children, unfortunately, in some cases. But myself, like a lot of other people, have the opinion that if YOU, as the parents, need to depend on someone else (your older children) to raise your children (or you can't afford to raise them), then you ought to think of the futures of your other kids and yourselves, and stop having children.
That's just an opinion though. It's neither right, nor wrong.
I do believe there is a fine line between children helping out around the house, and what families like the Duggars do, raising their brothers and sisters. (Of course, the media only shows what they want you to see too).
Anyway, I, personally, agree that children having chores is not just beneficial, but essential to their development. My kids stand on a chair and put dishes in the dish rack after they've been rinsed, they clean their own toys, and like to "help" sweep the floor. That's great, they're helping, but they're not doing the majority of the work. We still are, and as they get older, they will have more responsibility. That being said, however, I won't turn around and teach Isaiah to dress Zoe, change her diapers, administer ativan, drive a car, take Zoe to therapy, sit with her in the hospital during her admissions and surgery, teach her the ABC's and how to count. I won't teach him to do her laundry and how to properly stain treat her clothes. I won't teach him to feed her when she needs assistance. I won't teach him to care for Zoe because Zoe is MY CHILD. She is MY RESPONSIBILITY. She is Isaiah's SISTER, not his responsibility. If I couldn't take on MY RESPONSIBILITY of raising Zoe, then I would not have had her. If Isaiah decides when he is an adult and I can no longer care for Zoe that he WANTS to take on the responsibility of caring for her, GREAT! I will let that be his decision, but I would NEVER force my children to raise my children.