hi. well i have owned many dogs and i have experianced this many times, even with my dog and cat. what i would do is to first put a leash on the bigger dog, then put down the food bowl for the older dog first, after that put the puppies bowl down on the opposite side of the pen. you should have a couple of dog treats in your pocket for the older dog, you can use the hose or a spray bottle of cold water too. when the older dog goes to eat the puppies food say NO in a stern voice and pull the leash back towards you. when your dog turns around to his/her own bowl reward him/her with kind words and a treat. that way the dog will learn that bugging the puppy is bad and eating his\her own food is good. you will prolly havta do this for awhile, but you will hopfully soon see a difference.