It's pretty sad you said "Hopefully, YOUR children will be willing to do that for you some day." What if they don't have children? What if they can't have children? Just something to think about. It's cool that son is helping, doesn't have to, but cool I guess. Doesn't change the contract Garry.
It doesn't matter how people should be treated, just follow the damn contract and required laws. It's a business. Just follow normal protocol for adding a new tenant and raise the rent if it means being closer to value of other properties near you.