I was at the instistute in SLC last week too, and I've been consistently for the past 9-10 years, sometimes taking teacher training, sometimes working, and for the past two years, taking my kiddos.
There is tons of stuff for advanced players- it really is amazing. They bring in masterclass teachers from everywhere (this year, I got to watch Jenny Oaks Baker) to teach the advanced kids. They have their own activities, and its not uncommon to see 12-13 year olds playing major concertos- Mendelson, Bruch, Lalo etc. They also recently started an intense "chamber days" program that starts a few days before the formal institute where the kids in quartets have 4 rehearsals daily with members of the Fry Street quartet, then they get to perform during insitute. I was super impressed witht he Fry Street this year, and heard a couple of their coached quartets, and for middle/high schoolers, I was impressed. I heard one student quartet do Beethoven Op 18, and one doing Dvorak's American, so the level of music is pretty high.
As far as violas go, we've had the Preucils for the past few years, and everyone has had wonderful things to say about them.
It really is well organzied and well run. There's two concerts every day, and they really are well put together. I haven't had a bad experience with a teacher yet, and I've been observing teachers for years. My dd cried at the end of institute this year, and told me she wished it would last a month!
And Miranda, you definitely wouldn't be the only one with four kids!