The Gradual Height part makes it so that your child and teen sims grow, every day, at midnight, which is pretty cool, I admit!
This marks the return of shorter teens from the old mod! This time, it also shortens your existing teens! The Gradual Height Change is sort of a slider, but it's also not simultaneously. Gradual Height Change and Randomized Height for NPC's(added in version 0.05)
If your sim isn't wearing any of these, the slider won't have any effect, unless the slider is being applied for the first time, where any clothes will be affected.