Supposedly, kids under five and animals can see Al (and Sam for who he really is) because they're pure of heart and can only see the truth. sounds hokey, but that's how they explained it in the show.
Also the "mentally absent", some elderly people, and people who neurons and mesons are on the same (or close) frequency as Sam's can see them.
Does that help?
If you think about some of the explanations given it's not quite so hokey. The idea is that kids of that age are too young to have been corrupted by the world and see it with innocent eyes. Obviously animals can't be affected by scepticism and they aren't brought up to believe that the world works in a certain way and thats the only way things are. So if you delve a little deeper it's not so bad.