How dangerous is it for a small baby to have pneumonia?

It can be. It *can* be very dangerous for an infant to get pneumonia. Pneumonia can be caused by bacteria, viruses, and other problems. In most cases, there is a straightforward medical treatment and most babies are significantly better in a few days. Rarely, though, a baby can have a serious infection which is difficult to treat and may cause long term complications or even death.
Varies. In some it is rather mild and little worse than a bad cold. In some it can be deadly. The condition varies with the germ involved and other factors.