The great thing about my therapy setting is I am in the school system. Treating kids from the ages of 3-21 meaning the kids I treat will be with me for years and get accustomed to my phrases and therapy techniques.
The phrase I teach them is “fix your feet”.
When a child is W sitting, I will ask them to sit like me (showing them to sit criss cross or long sit) then I will say, “fix your feet” as they correct their position. After a while, I can simply say “fix your feet” and they will automatically correct their sitting position and slowly break the habit of W sitting. This also allows the child to not be physically corrected in front of people, we all know they hit certain ages where being corrected in public would be the end of the world.
In addition to correcting W sitting it is a good idea to incorporate activities that will engage the core muscles (The Importance of Core Control blog post has some idea) and increase bilateral coordination and crossing midline while playing, this will increase weight shifting which W sitting does not allow, they will naturally not be able to sit in that position and participate.