Hi C.,
Looks like I'm a little late in viewing your question - plus you've already received some wonderful responses, but I just wanted to chime in. I cannot imagine parents feeling that they are entitled to leave their kids at your home without arranging times in advance, and and without offering to reciprocate ! Every other parent I personally know just has to either take time off from work, or enroll their kids in a summer program, and wouldn't think of expecting someone else to fill in for babysitting.
I personally would be even more hesitant to leave my child in your home for any period of time, if I were in the postition of the other parents, considering that you home school and run a home business and most likely have very important lesson plans and things to accomplish during your day, and I would probably offer to take your kids off your hands for a day to give you a break!
But after years working as a nanny (now I'm a mom of a two-year old), I know how these things can happen. You probably have a very kind demeanor, and your home is probably very welcoming so the parents feel it's an open invitation, and the kids I'm sure love to be in your home, which even further compells the parents to want to think it's all right to take advantage of your kindness.
Many times when I was nannying, children would just "show up" to play all day with my "charges", which of course I never wanted to say no to, because it cuts down on the kids' summer fun with their friends, but then I never felt right when my employers came home, as they were incredibly lovely people, and the house was turned upside down because I didn't have time to sneak in those straightening projects I liked to do for them.
On top of that, it was never the lovely children whose parents allowed this to happen - no, the parents of those children would check in with me, very respectfully, offer to pay me themselves for the time I spent also watching their children, and always seemed to be just as happy to whisk all the children off on an exciting adventure, leaving me paid time in my employer's home to peacefully attend to cleaning and other services I performed, or plan meals and activities, which I really enjoyed, and hopefully helped the house run more smoothly.
No, the children who seemed to just "show up" would often be the ones that started fights with my "children", or amped up the drama and chaos, or just were super unpleasant to be around, and pushed boundaries, negatively influencing my "children", and causing me the stress of tracking down their parents when I had to leave the property to do things with the children for whom I was caring. And no wonder those parents were happy to have their children away for long periods - fighting with my "children", rather than each other at home, which their parents I am sure had tired of.
I can't even imagine what it must be like when all that is occurring in your own home, which you have to then try to recover from whatever state the visitors have left it in, and try to regain a sense of peace and order, knowing very well the next day it will probably be "cycloned" again, and cleaned out of food and treats you have purchased for your own family.
I know how excruciating it is to have to be the "bad guy" and say no to parents, or even harder, to other kids who want to play, and none of our advice on here can truly make that easier for you. However, whenever people seem to think it's just fine to use your home and time like it is less valuable than their own, you have to ask yourself, "what are the messages I'm sending, verbal or otherwise, that make people feel this way?"
Perhaps you are so easy-going, and friendly with these parents, that it gives the impression you really have no important business to attend to that their children being over would interfere with - so I would work on boundaries, and finding ways to even cut off chatting with these parents with a friendly but firm "Oh, I have to go to pick up the ....," or "I need to check my emails and fill an order asap ..." so the parents really get the concept that your time is as precious as theirs, and begin to treat you as though your home and time deserve respect. If they are too clueless or self-absorbed to pick up on that, the next time a parent shows up with a child I would greet them, and then immediately look at their child with an apologetic face and say, again friendly but firmly, "Oh, I am so sorry Brian/Mary, but we can't have a playdate today because we are very busy doing x...." Don't even leave time for their child to get involved in playing at your home, because your resolve will fade, and it will be that much harder to send them home.
I hope this helps, as it sounds like your own desire to preserve others' feelings, which is a lovely quality, is making it difficult for you to run your own home smoothly! I always remind myself, though, that the energy and time you expend not wanting to inconvenience others, is actually taking something away from your own family, if it is inconveniencing you, and that it is much worse to take away from your own family than someone else's!
Sorry this is a marathon response, but your question really struck a chord with me and I feel your stress and pain - hope it will continue to resolve in your favor !
Love,
K.