Some links to explore – You Can Never Know Too Much!
Introducing Cats and Dogs
Kids and Cats – A Match Made in Heaven?
Introducing a New Cat or Kitten to Your Resident Cat
Litter Box Issues! They Can be Fixed!
Excellent description of how to retrain your cat to the litter box if there are problems, AND if you have already had a vet visit for your cat to be sure there is not a physical problem.