The shelter staff cannot tell you how your new dog will react to your dog, your cat, your guinea pig or your kids. An improper introduction can result injury or even death.
Introducing two dogs is best done on neutral ground. Keep both dogs on a leash and be prepared to separate them. You may want to have a fence between them. Introduce them gradually and don’t be surprised if they exhibit aggressive behavior. Often they are just getting to know each other and after a day or two, they will be friends.
The first time your dog sees your cat, your dog should be on a leash and the cat should be behind a fence or visible through a glass window or door. Some dogs may not react at all, some may want to play and others may exhibit aggressive behavior. After several days, if the dog continues to exhibit aggressive behavior, you may want to consider hiring a dog trainer.
No one can predict how your new pet will react the first time your two-year-old pulls its tail or reaches into the food dish while it is eating. Keep in mind, your new shelter pet may not be accustomed to children and children may not be aware of the potential risks unless you teach them.