DON'T JUST SIT THERE!
I come from a household where corrective, encouraging, commands were heard on an ongoing basis. Like, "Sit up straight", "Ain't isn't a word", "Slow down and chew your food" etc. Ongoing encouragement like this actually shaped my life and had significant positive impact in the overall ability for Bob, to interact with society.
This isn't exact science, but my guess is that at least 75% of the unwanted lines in our drawing come from trying to draw against the way our body is made. Once a child, or adult for that matter, is taught to use their hands, arm and wrist in a way that is the most easily controlled, an enormous jump in sophistication is noticed.
In my book, I spend time on how to sit, hold a drawing tool, and how to use the hand and arm in an efficient, ergonomic manner. If you are purchasing Simply Draw for a child, (No younger than 10 on average) you'll need to "encourage" them to follow the examples, "sit up straight," "hold your pencil like Bob", "Your arms moving the wrong direction," Almost all new students have a "wealth" of bad habits that just take time to work out. I have videos and illustrations that will help. It is so important, that I'm sure your kiddos will appreciate your kind admonishments when they grow up! I know I still do...