This is my standpoint:
If my sister (currently 12), DBF's kids, or my own kids one day want to live with us after 18 and are full time college students no rent is required. They will, however, be responsible for all of their other expenses. If they are they're working FT, rent would be required, depending on how much room I'm actually giving them. (Example: I'd charge less if they only have a bedroom vs if they had the whole basement, ya know?) as well as expect them to help keep the house. I will not be their maid, at all.
DBF's oldest wants to be in the military. If he needs a place to stay for a month or two in-between assignments if he goes that path, it'd probably be rent free as long as he was saving. That's more of a "thanks for protecting our country" benefit. If he gets married and is in the service, his wife would be welcomed to stay with us, but the rent subject would still be there like it would with any of our other kids.
What we actually do with the money would depend on our own actual financial standpoint. Ideally I'd like to save it for them whenever they buy a house. However, this would not be known to them since I'd want them to learn how to budget savings on their own.
ETA: Honestly, I kind of wish my parents did this with me. I (obviously) would contribute when I had some extra money, (I took DSis and her friends to the movies tons of times on my own dime, bought food, and paid a bill here and there). But I never had a "set" amount to budget for. Because of this, the first month or two living on my own when I moved to Boston I was in a big shock....now I'm back on my feet and plan on pulling both DBF and myself out of debt. (I found visiting home about 1k of medical bills I had no idea I had....it's been sitting on my credit for years now apparently....my goal is to pay those off first.)