If they're old enough to talk with you, they're old enough to "play" in the shop. IMO, then the only limiting factor is how good YOU are at teaching them and keeping an eye on things. They'll surprise you at how fast they pick it up, and as long as YOU keep them out of trouble, there's no downside.
IMO, don't worry too much about finding "kids" size gear. You need something that fits them in the sense that it stays on, doesn't inhibit the job, and blocks whatever it's supposed too, but otherwise keep in mind that they are constantly growing. If you have to keep buying them gear every time they have a growth spirt, IMO they'll become less attached to "their" stuff. OTOH, if they get to grow into their gear and THEY are the one's that wear it out, they'll want more. Plus it's a bit of a constant reminder to them that they are doing something "grown-ups" do. Some kids this could be an excuse to act out, but most in my experience will have a little more respect for the job. Kids are smart. When they use the same "real" tools that us grown-ups do, there's a learning curve where they learn not to bang on things, but once they realize they're doing something "real" with dad/grandpa/etc., they tend to shape up.
I don't like schooling other parents as I don't have it all figured out and every kid his his own quirks, but IMO, the worst thing you can do with a child is see them as "only a child". IMO, if you look at them like an adult in a child's body, it fixes soooo many relationship/behavioral issues. Their bodies and minds need to grow, for sure, but if you procrastinate the day that you see them as a "normal person", you'll be sadly disappointed in the opportunities missed. It works the other way too. As soon as you start seeing things as "only for kids," life gets boring. The only thing that really changes as you grow is learning how to make things appropriate for the crowd you are with (maturity), and learning how to do things better.
We see much of our life as work, but those same activities with the kids often make us their coolest friends with the best toys. Work and Play can happen at the same time, and it's a fantastic thing for kids and adults to learn to enjoy both. It's hard to catch-up when you haven't lived life that way however.