There are going to be dominance issues anyway. The dogs you adopt need to be family dogs. Never adopt just because your kids want dogs. They'll be moving out in a few years or less, so the dogs will be spending their lives with you, not them. You'll be doing all the work with them. Can you handle having three dogs by yourself?
I would adopt a dog old enough to be able to handle your 1 year old dog, but not a puppy. The 13 yr old dog may not be able to handle a hyperactive puppy.
I can tell you that GSD puppies are very clumsy, and your little dogs may not be able to handle that. Adults are much more graceful.
My GSD(maybe mixed, not sure) was a rescue. We thought she was fully grown, but the vet guessed her around 10 months. She was quite clumsy. Thankfully our other dog was the same size she was, so he could handle it.
Edit: Four of my five dogs were/are rescues, and the only dog I had that WASN'T housebroken was the one that was given to me as a puppy. My Rescue dogs were all housebroken.