I kept my cat in for a week but then he was so happy exploring the new house that he didn't seem that bothered about going out. Before I let him out for the first time, I did it just before he would normally eat his tea, that way he would be getting hungry and knows where and who provides it! When he did go out, I went into the garden with him and kept an eye on him. He was so busy marking the new territory as his own he didn't think about running off. After about an hour of wandering around he came back indoors for his tea as predicted. Cats normally only run off if they get scared and are unsure about the new environment so just try to make it as normal as possible for them and I'm sure they'll be fine. Good luck