I actually agree with @lookingforward on this one! Senior year is too late, the local high schools here say to consider finances in sophomore or latest, junior years. The last thing you want to do is work on your supplemental essays for Stanford or the why Vanderbilt essay during summer after your junior year only to find out the family can’t afford it.
“no clue about what they can actually afford that they need a reminder to at least assess where they are now”
You’re not going to suddenly get a clue, especially if you apply early, September is not going to help when the apps are due in November, they’ll be working on essays, recommendation and their courses. They’ll have to apply for FA and hope things work out. You can tell them all the things on the checklist, they won’t know they can’t afford a college until they get the FA package.