First of all, I'm not sure you can even get trained in a constructivist approach, can you? I don't have that training, for sure. That means I am just making it up as I go along with whatever information I can get from books, talking to people, trying, reflecting and trying again. Maybe we can figure this out together!
Here at Sabot, we believe that drawing to learn enhances understanding, and in order to be able to make thinking and learning visible, children need to feel comfortable drawing and have the ability to draw from observation. So it is important to keep the student engaged and not totally frustrated, by demonstrating the technical skills they really need for observational drawing. I always try to do that in context. That is, when someone needs a new skill to do what they have in mind, I try to show it to them within the work they are already doing. Children don't usually need much teaching. I try to show that noticing and observation is the most important thing in drawing, and I teach about making 'studies', or a series of drawings to try to capture something. The use of studies really helps children with perfectionist tendencies stop throwing away draft after draft, and begin to learn that every drawing, no matter how flawed, is still valuable.
Most important, after learning to notice, is to give people time to mess about and practice drawing.
I really believe that direct instruction in art technique leads to stilted, superficial work, unless the student applies and practices the technique enough to make it their own. I don't want to encourage techniques a la Bob Ross, even though he seemed like a very nice man. So when someone asks how to draw a horse, I might go get a book about horses and look at it with them, noticing out loud the form and shapes I see in the horses figure. I would ask the other children for help. Sometimes we might take a trip to look at real horses and draw them.
Sometimes it seems hard for people to imagine the fine line between scaffolding and instructing in a school like ours. I do want to loan skills to the students, but I have utter faith that just telling them something (or just demonstrating it) doesn't make much difference in the construction of knew understandings. In the studio, I try to offer the bare minimum of help, but enough to keep the person trying. It is just like you might start with moving closer to a child who is becoming aggressive, and then gently lay a hand on their shoulder, and then reassess to see if more support is needed...except with drawing you might talk about where you notice a dark shadow on the side of a ball, and maybe even show how to hold the pencil on it's side to create a wider mark for shading.
I know that this is one of those "it depends" answers, but there it is. It depends. I hope that helps a little. What do you think?