I just managed to finish the game with the livestock ending and I'm happy with it as it is, but if anyone is stuck where I was the answer I received was a bit confusing. I couldn't find a way to get Bart to move from Nate's house, not Gale's later on (that's when you're supposed to get a second youth potion and go get a spanking from Martha). At the earlier point where I was stuck, you gotta check on Martha's house (where the first kid, the one you rescued from the woods, is still recovering) and then talk to Tiffany during the day for another demon potion and she will tell you you don't need it and that now you can give life force by yourself to heal others. Then you must go to Martha's house at night and give 3 hearts to the kid to help him recover fast, and the next day his friend Bart will come to stay over with him and Tiffany will be able to drain her first target, Nate.
I posted this just in case anyone happens to get stuck where I was, as it is a bit counterintuitive that in order to get the kid Bart to move you have to cure the other one that lives in a completely different house...