My girls go to a sleep-away camp (though we are in California) for 2-3 weeks. The minimum age is entering 3rd grade (so, 7-8 years old). My older daughter is going for her 4th summer, and my younger daughter is going for the first time this year.
I have observed a few things about good sleep-away camps:
1) They tend to be pretty expensive, about $1000/week
2) The facility will be well-kept and clean
3) There will be a high ratio of counselors to campers, and the counselors will have attended training prior to the camp starting
4) Good camps are usually known for their great cuisine (vs. the camps where they serve slop food)
5) They will have a great rate of campers returning year after year (as well as counselors)
In the parent handbook for our summer camp, it says that in terms of homesickness, yes, any child who has a happy home life WILL be homesick. That's just how it goes. We were advised to speak with our daughter about it before bringing her to camp, but also assure her that we knew she would overcome it. Also the handbook advised against making any deals, such as, "If you hate it, we will drive right to the camp and pick you up." That would undermine the process, obviously. Our daughter WAS very homesick, and after the first week, when she was able to call home, she was crying and it was all I could do not to cry, too! But when we went to pick her up at the end of camp, she was all smiles and didn't want to leave! So, even though they do get homesick, it's okay, they get over it.
But I would say that 7, almost 8, is probably the youngest you'd want your daughters to be for a 2 week camp. If it's just a few days, they could be younger. Be aware that because they're different ages, they would probably not be in the same group together, so it might not matter that they're at camp "together" - they won't see each other much.