It would seem that most kids art sites out there are geared more towards art play than art training,
and whilst i can understand that art play is sufficient for most,
there is the occasional young child who wants more than that.
I say if they want to learn lets let them
First things first
what do we need to foster, encourage, nurture and help them expand upon?
your child's imagination
(not just the ability to paint/draw things they can see or wish for
but the ability to see what others might miss, to see beyond what is available by a casual glance)
hand eye coordination
(sounds simple but you would be surprised how many fail to practice and hone this)
(not just recognition of colors but how they work together,
how light and shade change our perception of them)
(well you get the idea)
some of the most useful "lessons" in art come not from
exercises with drawing materials, but from observation and conversation
laying on the grass with your child "cloud watching"
pointing out shapes you see and encouraging them to find shapes of their own
pointing out interesting objects, it can be anything, an interesting stone, a shell on the beach (anything they can touch,feel, physically explore)
discuss the texture, the shape, the way the light hits it, the patterns the shadows make.