It's your house. They're YOUR horses. Did you offer to let these kids ride your horses? If not, then these are little brats for even EXPECTING the "treat"!
That being said. I got my first horse almost a month ago and the first thing my teenager said was "When do I get to ride her?" I answered quickly, without thinking and without regret "You're not. She's my horse. I've waited 39 years for her." (Keep in mind, my daughter is also inexperienced at riding and at caring for a horse AND she never listens to anything I try to teach her-she IS a brat).
I guess I'm just a selfish li'l b****. Yup, I am.
They don't seem to understand that the horses aren't mine to begin with, so there is no way in H*ll I would put any of them on one of my instructors horses. (My instructor said I could bring people out to the barn so long as I supervised them and did not put them on the horses. Hence my attitude against teaching them how to ride.) On top of that, half of these people don't need to be around a 75 pound dog, let alone a 1200 pound horse.
To the OP,
Try looking online for position diagrams. I'm sure there are a few out there that could help you explain proper riding postion. There may even be a few on guiding the horse with leg pressure.