Bex, I'm 99.9% sure I know you from elsewhere...
My wife just made a comment when I mentioned your situation and I think that it's worth considering. Work to Live, don't Live to Work. Then there are also the priorities of Family above all else, then Work.
I'm somewhat envious of where you are, but there is a lot to be said for being at home every night, although the summers are certainly hard work where I am.
If there are ways of improving your lifestyle at BA through part time and/or parental leave and being able to stay there, I'd certainly recommend it. From my point of view, you've got almost certain job security in a legacy airline.