Though groups like the American Kennel Club make very specific designations about breeds and breed hybrids, there continue to be new breeds and breed variants developed all the time, as well as "up and coming breeds" not officially recognized by kennel clubs and experts. (Many dogs not purchased directly from breeders are also mixed-breeds of some kind, or have less than purebred ancestry.)
When large dogs eliminate, they eliminate a lot more than small dogs. More goes in and more comes out of large breeds. Which is something to keep in mind if your dog will live inside - especially if you live in a high rise apartment and he will have to be litter box trained (there is always going to be more to clean up).
Most all dogs will get along well with children if they are raised with them. However, some breeds have a protective streak in them and may become aggressive with children who get to close at meal time (although this should not be tolerated), or aggressive with children they have never met. These breeds will likely see the children as being underneath themselves in the "pack order" and may try to dominate them.
That’s why we need breeders of purebred dogs today. People who breed to preserve dog breeds are usually hobbyists. They may participate in dog shows or companion/performance events with their dogs. The dogs that they can’t keep are usually placed in pet homes. Yet cities and state legislatures are passing laws that can make it virtually impossible for smaller breeders to continue this important work.