I am the daughter in this. Same circumstances and ages.
You sound totally unrealistic and unused to looking after your grandkids and your DD has tried to make the weeks easier for you.
Taking kids those ages on those day trips and more within the space of a week or two would be disastrous. Tennis club at those ages mainly consists of watching their peers miss the ball five times then they miss the ball five times, on repeat. They'll be raring to go after lunch.
My parents (and in laws) have done exactly the set up you describe and love it. My husband and I take our kids all over the world, we really test them but I wouldn't do those three day trips in the summer holidays in the space of two weeks - misery.
This way you get no stress, manageable time with them to build your relationship. Invest that time, on your DD's terms. She has not done this to annoy you or to take away from your time with the GC.
Stop and think for a moment about who your DD is and why she has done this. To help you? To protect you whilst you build the relationship so you're not tired and stressed? To make sure you're looking after 2 happy relaxed children that are not testing boundaries with you three hours into the day I.e. by 11am? Maybe because she knows that if routine loosens too much she'll be dealing with grumpy overtired children who won't go to sleep and she can't do that whilst being a good employee and not working in the evenings so she can hang out with you?
The way you mention the six hour drive is telling. You see the Wednesday as a lost chance. My parents would be out to the shops buying ingredients to cook a lovely meal for that evening for the whole family. They'd probably also cook a few extras to fill my fridge to make my life easier. I'm 42, my parents are in their 70s and I've lived in London since I turned 18, so no apron strings here, just them meeting a manic family on our terms as they can flex as retired and also, wanting to support me the best way they can. Their relationship with my kids is wonderful and they have sleepovers whenever they can when we're up there and we disappear!
It's also lovely to see the grandparents enter the kids real lives. Dropping and picking up at clubs, seeing them interact with their friends, hearing about their exploits first hand. There is more than one wa@y to build a brilliant bond and going with the flow now will reap benefits in the future.
I'm writing all this not to criticise but honestly, in your DDs position, I'd be so upset that an innocent action designed to do the best for the kids, my mother and their combined needs, caused my mother all the feelings you describe.