Well - when human females ovulate, they only release one egg. That egg can be fertilized by one sperm. Therefore each child has one father.

When a dog ovulates, she releases many eggs. Each egg can be fertilized by one sperm. If the female dog mates with more than one male, then each dog she has mated with contributes sperm. Each egg can still be fertilized by one sperm, so it just depends which sperm cells reach each egg! Each dog still only has one father.

Believe it or not, it can happen with humans as well. Fraternal twins occur when a woman releases two eggs. There was actually a case when a woman gave birth to fraternal twins - one black and one white!