I am interested by the 'talka bout it / don't mention it' dilemma. So much depends both on the relationship you have with the parents, and how likely they are to be able to change things around. You certainly don't want to stir things up if there is no chance of change, and you just make them feel worse.
My son & d-i-l have managed to sort out their hours so each collect from school one day a week.
I wondered (and this has so many variables!) if you would be able to do 2 days a week for 3 weeks, then the other GPs, when they stay could do those 2 days once a month. I know that, at a similar distance, I wouldn't want to do twice a week, but might manage if I got a week off every month. However, with whatever needs to happen during school holidays, and other commitments, it might not be do-able.
When you have read and digested all these comments, you might be able to think of a time (as suggested by mostlyharmless) when you would expect your dear GD to be coping better. And that might set a time scale for speaking to her parents. That doesn't mean ignoring her needs, but it does mean setting them alongside others.
In the meantime, you are her rock.