Suggest that they save whatever money they'll be saving by living with you. Also, even though you are Dad, and love them greatly, be sure to give them shit for any unnecessary purchases while they're staying with you. My buddy was letting some people from work live with him while they "saved up to buy a house." He wasn't enjoying the extra company and they didn't seem to be trying too hard to save for, or even look for a house. The final straw was when the dude bought a freaking non-running project car for $5,000. Now they're back in their own apartment.

Anyway, of all of life's surprises, this one isn't so bad. Hope everything goes as smoothly as possible.