From your description, I don't personally think a behaviourist is required - unless things change from the current situation, that is. The fact that Porter is growling is quite understandable since he's had a few painful encounters, and it is also a good thing. He's giving warning that he is uncomfortable and warning is a very good thing to have
Moving Porter's sleeping place is a very good idea and should help to remove the cause of the problem. You do of course need to supervise all interactions between your kids and Porter, especially since he's giving off warnings. But I'm pretty sure you're doing that anyway. I'd say keep a very good eye on the situation, do your best to ensure that Porter's 'space' is respected and see whether things improve as Porter regains a bit of comfort. Keep the behaviourist in the back of your mind though, should the growling continue or become more frequent.