Upstairs downstairs and thomas and sarah what baby did they have and how did it fit into the followup series?

This is still bugging me. Asked yesterday about this and was told about sarah's stillborn child which was on a recent episode but she left pregnant ..so what happens in thomas and sarah..what baby is it. and i am remembering one of the other staff went into the follow on series who was that,,I am thinking it was rose? Not sure why I am thinking that as I also sort of remember she ended up in tenko. Any help appriciated after all it is my birthday..lol