Patey,
I know how wonderful of a mom you are are, and with children as precious as yours, I understand the feeling of wanting to keep them close. I have a different perspective, that doesn't mean yours is wrong. I will just give you mine though in case it helps

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I don't trust everyone with my son, hardly. But I have and will continue to let Austin have his independence in certain situations. He spends the night at his best friend's house, and his best friend spends the night at my house. I trust his mom and she trusts me. I am very grateful to have this kind of two way street for a couple of reasons: it makes my son VERY happy to have that independence away from home, and I feel good about it being in a safe place with someone I know will take care of him like his own son. They do things a little differently at her house, and I think it is a good thing that he sees that things don't always happen the way they do at "our house". And I think it's important for both of them to have that private time as friends to share fun they just can't share with mom and dad. Secondly, it is very helpful to have an allie that I can call and that she can call if we ever need a favor. I don't even like to call it babysitting because it is so not a chore.
Secondly, he has gone to Utah twice without us, and once to Tennessee - all three times supervised with someone in Calvin's family that we trust. As a result of these times, he is a very independent and already a diverse little dude with a ton of courage. He's ready to go on Kid Nation! (lol, just ask him). And he has some GREAT memories of times spent away from home. It was hard to be away from him, of course, but I always knew it was a little sacrifice on my part to give him that for himself. It's made him much more confident. Of course I worried and had to keep convincing myself of all this
Last year he went to church camp for a week on his own. That was a little out of my comfort zone and I was a worried mom up to the time he left and during that week. I asked LOTS of questions before he went. He loved it! He cried when he had to leave! He can't wait to go back this summer.
I spent the night at my best friend's house and my cousin's house all the time growing up and I went to church camp every year and those are some of the best memories I have. We also had lock-ins with our church and those were great fun. And of course there is after prom - those were kind of corny, but still well supervised!
Sorry for the long winded-ness, I just wanted to give you some examples before I explain. My opinion is that it is essential to a child's independent development that they have time away from the home in a safe setting (that you feel comfortable with), and essential for your eventual sanity when they do start leaving as they get older. It builds their confidence and yours. Even if you just do it a little bit at a time until you both feel comfortable. I think it will also make it a little bit easier when they have to fly the coop when they get older to go out on dates, after prom, to college or school trips.