As long as a child is old enough to know how to treat them and is taught how to play with dogs a Whippet is a good choice for a child. A very young child should never be left alone with a dog of any breed and interaction between a child and a puppy must be closely supervised, for the protection of both. They are rarely much good as watchdogs. This is not generally a very vocal breed, and some hardly ever bark at all. The most common reason for a Whippet to bark is that he has learned that he gets what he wants by barking.
Whippets are too small to be at all useful for protection.
Though most Whippets are not used for that purpose today, they are still bred with that instinct in mind, and the ones who are coursed and raced are bred specifically for this prey drive. Very few Whippets are born without it. Even if your dog is raised with your cat and loves and plays gently with it, if he ever sees the cat running outside, it won't be his beloved Muffy, but prey. Some owners do keep them together successfully, but our experience has not been happy. If you do have a Whippet and a cat, we suggest never leaving them alone together unsupervised, and certainly you should never put the two outside at the same time.