I think all you can really do is explain the importance of the essay, based on where they want to attend and check in to see how they’re coming along. If they’re not making progress, ask them a few questions to help them get motivated but that’s about it. At the end of the day, you can’t want it for them more than they want it themselves. If they’re serious about attending a selective school that values essays, they’ll get on it. If they’re not, then explain their actions or inactions will lead to a different type of school (which might be perfectly fine). There’s a difference (a big one actually) between having the ability to gain admission into a level X school and wanting to attend, go through the grind. Some just don’t want to. Not their thing. Nothing wrong with that.
I only take issue with those that want a certain type of school but aren’t willing to do the work. Doesn’t work that way.