Should 2 teenage boys pay rent, if both have jobs. we supply food, elec, tv,water,etc.?

we have 2 teenage grandchilden that we have had for abot 4 years. we just bought a 6 bedroom house and made the upstairs into an apt for them. this way they can have friends over and learn responsibility as well. i told them the other day that they were going to have to pay rent to cover food,elec,etc. you would have thought i had slit their throat. they both hav e jobs and ii think its only fair that they put in a small amount to help out, let me hear your opinions. one is 18 and the other is 17. the 17 year old has 2 jobs.