"I promise I'll take care of him!" Children mean well, but they cannot accurately gauge what is involved in taking care of a dog. Though you should encourage your children to participate in pet care, do not assume that they will be able to manage that responsibility on their own. Even teens may not be completely reliable.
Every dog and every child are different, of course, so be sure you've carefully assessed the situation before putting a kid in charge of an animal. Keep in mind that most children will need adult supervision while caring for their dog, most of the time.
As the parent, you have to determine which dog-care tasks your children can handle. Here's what you can reasonably expect: