How safe are large dogs with small kids?

Depends on the breed. Many large dogs are better than small dogs for children due to biting and personality. However, a large dog bite can be much more damaging and even fatal compared to a small dog bite. I recommend choosing your breed carefully. Golden retrievers tend to be large dogs that are great with kids. There are many great books about selecting a dog for your family or consult with your veterinarian.
It depends. Dogs, regardless of size, can be dangerous around children. Proper training, including bite inhibition is essential. Children must always be observed around any animal. Never allow your child to approach an animal who is eating or sleeping. Older or ill pets may bite a child. Teach children to respect the space of the animals in their environment, and to never approach an unfamiliar animal.
It depends. A small child should never be left alone with any size dog. However a well trained large dog should not be a problem if the child is not alone with the dog.