I had my eldest in an art class offered by a professional painter, art teacher and homeschooling dad in the area, back when she was about 5. This guy had a reputation for a very engaging style, was very charismatic and the kids loved him.
He did a lot of cartooning with the kids. They loved this. They worked with a variety of media. They learned to draw contours, to create perspective, draw lions and airplanes, paint in backgrounds, draw horizon lines, doodle with textures and so on. My daughter did a series of 8 classes and enjoyed them.
Then, about four years later, a local artist began offering art classes. I knew her socially and really liked her holistic philosophy. Without even really thinking about it, I registered my three older kids. They seemed keen enough.
These classes were SO AMAZING! My kids continued in them for three full years, until she stopped teaching them to pursue her own travel, educational upgrading and such. I've continued to beg her annually to start teaching again. She didn't "give art instruction." Instead she created an environment where the kids were inspired to see things differently, to experiment with the media and tools she was helping them learn to use, to develop their own ideas, their own sensibilities, to be able to talk about their thought processes, to appreciate art everywhere, to create and enact art everywhere. They were given just enough in the way of constraints that they were inspired to turn their thinking upside down and inside out and push at limits and assumptions. They were given just enough in the way of guidance and inspiration that they were able to generate their own ideas and move forward with them.
If I had it to do over again I would have kept my eldest out of that first "art instruction class." He wasn't teaching art. He was unwittingly teaching copying and emulating and conformity, even though I wasn't aware of it at the time. When you talk to "real artists" who have gone to art college and pursued careers, they talk a lot about spending time unlearning all their assumptions about how to do art, and getting back into the unconstrained creativity of childhood. I think they're talking about unlearning the sort of things this guy was teaching: how to do things right, in a way that pleases others and passes some sort of unspoken standard for prettiness and accuracy of execution, not make mistakes, not take risks.
A great art teacher is worth her weight in gold, but "how to do art" instruction is entirely the wrong approach, IMO. It can put the damper on a child's true creativity. Can you observe the classes and get a feel for how much "how-to" instruction is occuring, vs. how much supportive facilitation and inspiration?