No, you would be messing with their heads if you made excuses for him.
You COULD tell them that he loves them as best he can, but because he isn't well he can't be the dad he would probably like to be. I think it's important for kids to feel that their parents do love them, even if they are unreliable. It's not too early to teach them a little bit about detachment--that sometimes people we love will disappoint us, and that it's OK to feel sad but not to think it's our fault that it happens, or that they are doing it on purpose to make us feel bad.