Anna, what a scare, you must have been terrified when you saw them in the street, I know I would have been. I agree if they once found out they can do that, it will happen again.
I think the outside pen would maybe be a good idea, one can get pretty tall chain link fence and make it big enough so they have some room. Our son had a Rottweiler and that's what he did, Bear was in there during the day and he put a shade thing on top and when they got home from work he would come in the house. I don't think the chain link fence would be that expensive and one could do that themselves.
Good luck in whatever you decide and I am so glad both of them are O.K.
Jackie, Miss Daisy and Perry