I have a boxer pitbull mix who fits all of your desires. I mean she isn't the best with kids but I think that's cuz I was in college when she was a puppy and none of my friends had kids so she wasn't used to them and they made her nervous when they would run up to her being loud and chaotic. But over the years she's gotten much better and more tolerant when she is around them. Anyway pits are good with children, look intimidating but are really loveable and protective, easily trainable, low shedders, have tons of energy and could run for miles. And there are plenty in the shelter! Boxers are pretty similar but are a little more hard headed and therefore harder to train. That being said, every dog has its own personality so even though there are breed generalities, every dog is different. Some may have separation anxiety, others are hyperactive, others are super lazy, super talkative, very shy, or guarded and even stubborn. There are other dog breeds I could recommend too but may not fill all your requests. I have groomed dogs for 7 years and know all sorts of breeds but like I said, it still comes down to personality. So let me know if you're interested in other breeds I think could work for you and why!