No offense intended but seems very harsh to me as well. I have been lucky to have great families over the years but if I put out a letter like that with so many restrictions i think that everyone will walk away. Its really long and i bet most of them wont fully understand what you need. If you have a family or families who are dropping off whenever they want, you should address them one on one to discuss it. I get that your schedule has to chnge because of school dropp offs. Maybe just a letter saying, dear parents, as of xxxxx, I will be doing drop off and pick ups at school. we need to leave the house at XXXX in order to arrive on time. Therefore your child must be dropped off prior to this time. if you cannot make it on time, you are welcome to wait for us to return. Please note that weather permitting we may not arrive home directly. In such instances, you are welcome to contact me on my cell and arrange to meet us.
Then just add the same thing for pick up.

Just to play devils advocate, if for some reason you had to leave a few minutes early for school drop off for some reason and the parent arrived after you left but before your schedule time to leave. By your rules that parent could deduct your pay for that day because you weren't there when you were supposed to be.

I also think the last bit about putting the kids needs above the parents may not sit well with your clients. Good luck