Biting children-Cocker Spaniel

Question
My 4 year old female (spayed) golden cocker spaniel has bitten my 18 month old son for the first time. She has been wary of him for awhile (since he has been walking) and I have been very careful not to leave them alone together and I stop my son from bothering the dog etc.. She hasn't caused much harm to him (a tiny cut on his nose) and her reaction seemed to be caused by him getting too close to her and she was warning him off. She has the same reaction to dogs who sniff around her for any length of time and has growled and snapped (although not bitten) at my 2 year old neice for standing over her. My dog is fine towards me and my husband (abd other adults) although she can be protection of her food and toys so we feed her away from everyone. Clearly I am concerned about her behaviour towards my son and although I don't want to rehome her I am worried she will do this again and the consequences could be worse. Any advise greatly appreciated.  

Answer
he key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm  Also, follow theirlink to their page on dogs and children.

At 4 years old, the dog is mature and bad habits are well established.  If you are able to change her, it will take some time.