So many families bring a dog into the home for the kids because it's a wonderful  experience - it can be. It can also be a bad experience for both parties.

Good relationships between children and dogs don't just happen. They develop over time. Each participant learning what to do and what not to do.
Parental guidance is essential at all stages from new puppy onward. Responsibilities assigned to your child/children change over time.
A 9 week old puppy has different needs and skills than a 6 month or 1 year old dog. A 2 year old child has different skills than a 4, 8 or 10 year old.
When bringing in a new puppy or adopting an older dog, think of what the dog needs on a daily basis and assign age appropriate responsibilities to your children.

Is one responsible for walking the dog? 
That requires knowing how to and be willing to pick up poop! Walking a new puppy is very different than walking a dog who has learned not to pull on leash. Does your child know what to do if he meets another person and dog? What if your puppy starts barking, pulling, biting at pant legs? 

These are only some of the questions you need to consider. Yes, you may have gotten a dog "for the kids" but parental guidance and responsibility play a big part until the puppy has learned some skills and developed some self control.

Lucky for parents, there some wonderful resources for families with dogs. Visit my Resources page for links to two such resources.
Doggone Safe: Help Dogs and Kids Build A Lifetime Bond.