I think Brockhole looks good and not been there yet. Got a playground which is good, things inside, somewhere to eat and also the boat if we wanted to.
Tarn Hows looks lovely (pretty sure I've been there) but not sure it will keep little ones entertained?
I would vote against Haverthwaite simply because we do it every time we go to JTs as we can walk to the station from her house! Happy to go if that's what everyone else wants to do though.
So I think my vote is for Brockhole but happy to be persuaded otherwise!