DD did one last year. She was grade 3ish at the time but with very good sight reading. It was a bit beyond her at the time & I wished we had waited a year or 2. So grade 6 should find plenty to stretch your DD! I would try one. They tend to bring plenty of different recorder sizes for kids to try which was DD's favourite bit.
Yes, she has Grade 6, but is taught well beyond grade, if that makes sense. Unfortunately summer course is while we"re on holiday, otherwise she would have liked to audition. Wanted to last year, but nearest day was during (early) GCSEs.
Yes. To a certain extent it depends upon who else is there. And what level your DD is at (grade 6, right?) They normally get very good people to lead them so it should be possible to learn a lot even if most of the other people there on the day are less experienced. Will your DD be auditioning for the summer course?