I agree, with Suzie you need to inform them now. They may leave...but odds are they won't find a spot for under 2 years in the near future. Especially if they don't want to pay for summers.

You look bad to wait and tell them just before their end date knowing they want to stay on. Your reasoning is that you have already filled the space. You can't wait a few months and say oh I filled the space long ago I just never bothered to tell you.