No one has suggested that children should not do the work themselves or that they should be encouraged to plagiarize. I wonder if you read my words carefully. I never suggested that anyone do the schoolwork for his child.
Do you have children? If you do, their teachers will tell you that it is essential for parents to participate in homework--talking about the assignment to make sure the child understands and completes it, giving feedback, answering questions, providing appropriate materials. How you get from that type of help to plagiarism is unclear. Kids learn better when their parents reinforce the material they encounter during the school day instead of simply abandoning them. Helping a child learn is very far from cheating.
Your second paragraph has no bearing on what is going on here. Adults are being solicited to give the kid ideas for a design that the kid will pass off as his own. That is the essence of plagiarism--passing off as your own ideas that others came up with. Here's a thought experiment: what if rooster wrote for the teacher a short statement on how the kid did his homework, how the parent solicited a web community to get ideas. If I were the teacher, I would give the kid an F for cheating.