I second ColinsMum's points here. I've never heard of a situation where kids were thrown into an orchestra before they'd at least been offered the opportunity for instruction to reach a basic competency level first (though it wouldn't necessarily have to be 1-on-1). For example, in my school (in CA) all kids were offered violin in 4th grade... that was your only choice. You could take it all year, and if you stuck with it you could then progress to the Honor Orchestra, which was made up of 5th and 6th graders all over the district, playing the orchestral instruments of their choice. That was their first exposure to a group setting, after basically working on violin on their own for a year. I'm not sure what goes on in my DD's school yet, but I have noticed a teacher comes to the school for 1-on-1 instruction for violin for 3rd graders.
I'm not saying your kid needs to do the basic stuff for an entire year, but she also just started, didn't she? If your child is vaulting over that basic competency step and landing with a bunch of much older kids that had that already, that could be a setup for failure. I wouldn't want my DD, who battles perfectionism, trying to measure herself by their yardstick.
If she's already learning guitar, I wouldn't think violin or flute have much more to teach her. Both are note-at-a-time instruments, and violin is just another stringed instrument besides, just less versatile. But piano? Definitely. Anyone who wants a really deep understanding of music should learn the piano at some point.
It looks to me like she's so passionate about music that she wants to go broad and deep all at once. At this stage, I'd applaud her enthusiasm, then encourage her to go deeper first, and help her broaden her horizons later.
