I remember my first blackhead. I was on the verge of being a tween and it was clear in the mirror before me. It was not a random speck of dirt waiting to be wiped off the reflective glass, but rather something conspicuous that had appeared overnight on my tiny face. If this was a movie, the director would have focused in to find not just one tiny blackhead, but three cursed pockets of dirt directly in the middle of my chin, end scene.
1 - Don’t point out their flaws ~ It's really easy to be frank about this and state the facts. Think about when you were little, a tween, or whatever...would getting blackheads or "your changing skin" have meant anything to you? It doesn't to them either. This is less about skin and more about lessons...you're continuing the education on hygiene you've been teaching them all along. Remind and reward them as they're making it a habit. Few things in life are more successful than good old fashioned positive behavior reinforcement!