While they are affectionate with their own family, the Tibetan Mastiff is not a good family dog, being far too large and too protective. He is likely to mistake the intentions of their families friends. They can live with sensible older children but care should always be taken.
While many dogs are traditionally thought of as being good with children, all dogs and children need to be taught to get on with and respect each other, and be safe together. Even so, dogs and young children should never be left alone together and adults should supervise all interactions between them.