New St. Bernard owners?

We are a family with four children, two ragdoll cats, an elderly small dog, and we are considering adding a Saint Bernard. I am looking for tips, advice, etc. We have a very big fenced yard, large home, and experience with large and small breeds, but the St would be the largest we have had (mostly German shepherds previously)

Are they typically good with cats? Small dogs? Children? Any info appreciated as we do our research.