RE: Explaining to Children Why They Cannot Have a Dog
Paper training is great, but nothing is better than taking them outdoors and allowing them to do their business outside. Paper training is definitely advisable to those who live in an apartment and are unable to come home early. However, this does not mean that they are not to take the dog outside because dogs still need their exercise and walks.
Back to the question, I think it's fine for children to have responsibilities. But if the parents aren't able to supervise them and they're very young, it's rather not advisable for them to give their child a puppy.
A good test for children in order to determine whether they are prepared to take care of a pup is to give them a book to read or have them do some research on how to raise a puppy. Once they know enough, ask them to make a guide book (or notebook) on the important points to raise their dog for the next 3 months.
Also, it will help a lot if you could find them a neighbor's dog they could help walk. This will show them part of the everyday routine of a dog owner. After all, it's a lot easier to show them why they can't have one now than to explain it to them.