I like the idea (and I have done this myself) of saying 'we are going to my parents house straight after, every week sorry.....'
Or something like that.
I did something similar when 2 girls who lived half a mile from us, kept trying to cadge a lift back to where we lived (from the senior school that they and my daughter went to.) I gave them the odd lift back when it was raining, but they kept expecting it, and one even said (after a few weeks,) 'can you drop me off at my nan's?' Her nan's was a 5 mile round trip out of my way, in the opposite direction! And she expected this twice a week. Cheeky cow.
So after about 3 months, I said 'sorry, I have to pick up my cousin's kids from school as my cousin has a job that doesn't finish til 5pm!' One of the mothers properly slagged me off at the school, and said 'she is LYING, she just doesn't want to give my Kayleigh a lift home!' Like her daughter was entitled to a lift from me! She could drive, and had a car, but never, ever took her daughter to school OR picked her up. And she most definitely never gave my daughter a lift.
Cheeky bitch!
All this said though, it does seem a bit daft to not halve the amount of time and petrol and driving you have to do. Your choice though.............