The other thing to look at is if this child is a friend of your son's what would he be doing for that half hour if the friend wasn't there. Is there a way to make better use of the time such as having them doing something quiet for the last hour of the day while you cook - even playing games at the kitchen table.

If mom is on mat leave then it will depend more on what time the dad is off work if he is the one coming for the child and that will happen more in the winter rather than mom taking baby out. You could approach the family with the idea of what is the earliest you could pick up since our lives are getting busy in the evenings and getting supper overwith earlier would be nice and see what they say. You don't have to be specific about what your evening plans are - just sitting and vegging is an important plan but not one we share.