At the camp that my kids have attended (Blue Ridge Suzuki Camp*), if your child is under 13, you're going to camp, too. No drop-offs under 12. They have a "teen cabin" with an adult chaperone for the teenagers.
*Plug for Blue Ridge Suzuki Camp -- wonderful place, extremely welcoming. Not for prodigies. For normal kids to enjoy a really comprehensive experience with good faculty and a nurturing environment. They also accept guitar and even a few piano students -- helpful for when there is a sibling who is not a violin student.
I only know US programs.
The one that takes them the youngest is Heifetz Junior Program (age 8/9 is the youngest). There are several other programs that will allow kids who are 8-11 to attend, but they have to live off campus with a parent. These include Indiana University's Summer String Program, Center Stage Strings, Aspen, and to some degree Bowdoin. There may be some others, but those are the main ones I know.
Most of the advanced programs start taking kids somewhere between age 12 and 14, so in the upper ranges mentioned in your question, you should have no problem finding something. Perlman, Meadowmount, Encore Chamber, Morningside Music Bridge, Center Stage Strings, Ascent, Bowdoin are just a few of the options. There are also orchestra camps for this level like NYO/NYO2 and BUTI.
There are some other junior programs, but they tend not to be that advanced. For example, there is a program at Interlochen for elementary-aged kids, but only the advanced program (which is usually 8 kids) would likely have any value. The level is otherwise really quite low. Some of the Suzuki camps have advanced programs (post-Suzuki level) but they vary in quality and level.
Is this for this year? Most of the advanced programs required you to apply in January or February, so you are out of luck for most places this year.
I think there were 8 year olds there when we were there, but they’ll need a parent or other responsible adult around.
I don't think putting an eight-year-old together with substantially older kids in a chamber group is wise, unless they're really extraordinarily mature. My son, who is a 7-year-old 1st grader, plays in an orchestra with kids who are in 5th - 8th grade, and the maturity gap is meaningful.
(That kind of mixing also means kids get exposed to conversations you might not want them to have.)
By saying “advanced”, I refer to the equivalent of Rode caprices, Bruch concerto, Mozart 1/2/3 concertos, Beethoven sonata etc.
Participating in orchestra is a little distinct; the typical eight-year-old doesn't have the attention span, focus, or maturity of a child who is in middle school, much less a teenager in high school. Expecting them to participate in an orchestra environment with older students is not necessarily fair to them, the rest of the orchestra, or the conductor.
Participating in chamber music is a wholly different situation. Collaboration requires discussion, and expecting an eight-year-old to interact as an equal with middle schoolers, much less high schoolers, is almost certainly both unrealistic and unfair.
If I were you, I'd look for a camp that has excellent private lesson and masterclass faculty, plus a good selection of enrichment activities, instead of looking for orchestra and chamber opportunities.
We've had the whole span of kids from five year olds doing Vivaldi A Minor to older ones firing out Bruch, Lalo, Barber, Sarasate, etc. Our faculty even includes our very own Mary Ellen Goree! Get in touch with me and I can send you some more details.
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