Just a few ideas - First, show them the hand shape you're looking for by having them relax their hand over the knee and then lift it up, keeping its shape, in a relaxed manner. This helps them to see generally what it should look like. Second, show them the point on the finger which should be depressing the key. If they have "dented" finger joints, then they need to move up toward the nail. Tell them that they need to practice this way to make their joints "strong". Giving complements as they practice this and get better at it will go a long way.
Having been a child who was allowed to play flat-fingered, then being made by another teacher to curve my fingers, I will say that it is a frustrating process. Because of my memories, I do not insist that my students play with the tips of their fingers ALL the time. I try to be strict when we do scales and offer reminders during pieces. But I don't point it out every single time and correct it, if that makes sense. You don't want them to end up with too much tension as they focus so much on one thing.
These are just a few ideas. I have only had two students who tended to play with collapsed finger joints (one was a transfer, so maybe a bad habit). Most students have had no problem once I showed them the correct position and reminded them here and there.