I draw a lot with my cousin (he's now 7). I've been drawing with him since he was 3. At such a young age, I think kids need to just be encouraged, not taught. They can pick things up pretty well through osmosis, and I know that's how I learned when I was a kid, drawing with an older neighbor. Just drawing together, they'll pick up on things you are doing. No need to "teach", but you doing some simple drawing exercises will help.
Draw stuff from real life, have a sketch day at a museum. Draw something on request. My little cousin has already somehow picked up perspective and draws planes and trains in orthographic projection (top view, side view, front view). Plus he's now doing callouts and different viewpoints (ant view, bird view) and action lines showing movement. Have no idea where he's getting it from
I've never been too big a fan of most drawing books from kids. They just show shortcuts (ie. draw three circles, then connect them with lines to make a bird), and encourage copying and drawing by rote, not real sketching or thinking of the "why". I'm sure there must be better books out there, but I've never seen any.
Best you can do is just encourage. I buy my cousin a moleskin notebook and some professional drawing tools every year for his birthday. I think he likes how I take drawing seriously and for fun, not just like most adults who give kids a crayon to shut them up.
Another exercise I like to do with kids is to do drawing storytelling. Simple sideview drawings that I just makeup while telling a story. Ie. There is a car, that has a robot arm attached... it is made to lift up an elephant, to help it escape from the zoo....meanwhile, a plane is flying by with a giant ladder attached ...
(all while simply illustrating what I'm saying. Makes no sense but shows drawing as a way of thinking and is really fun, plus they can add things in at any time since the drawing is so simple and it's not about making serious art so there is no intimidation).