I've had a similar situation; although my job should technically keep me there till 4pm, I've arranged to leave at 3 most days, and go in a bit early, do some stuff at home, etc. My boss is great, but what's happened recently is that the person who I could count on to pick them up once or twice a week so I could make up the difference is no longer able to do so, with the result that my overall pace is becoming a good deal more frantic, with considerably less flexibility; I do sympathise.