I did this with my daughters. Both got certified at 10, and were clearly not done growing. I live in FL, and close to the water, so in most cases, they can get by without a wetsuit. The older daughter has a wetsuit, but not the younger one yet.
They both took their classes in their own gear, which I think helps to gain familiarity. Apart from the wetsuit, my biggest concern was the BC, as most out there are not designed for kids. If they are, there's a good chance that they'll outgrow them quickly. I opted to get them Zeagle BCDs. The oldest has a Lazer (now discontinued), the youngest has a Zena. Apart from a BP/W these are really the only ones I've found that can grow as the kid grows. They have the usual adjustability, but also offer different size sections that can be replaced. For now, they are both still in Smalls, but if they grow out of that, I can get a Medium shoulder and use with the rest of the Small BCD. I had to do similar with my own, Medium with Large shoulder section.