For the most part they treat him as if he doesn't exist. Occasionally Spike will attempt to sit in my lap even though Huxley is already occupying it. We end up having a battle, and I of course win. I push Spike. He pushes back and sometimes follows with a growl. Eventually, I knock him on the floor, and he moves onto other things. We had this same battle when I was pregnant. My ginourmous belly was non-existent to him.
Huxley only exists to them when he's making noises, like crying, or when he's providing food for them. When Huxley was first learning to grasp and shake his rattles, he smacked himself right in the head and started crying. Both Buffy and Spike came rushing over to Huxley. They knew it was a cry for help and came to his rescue. The only reason Buffy made it into the above picture was because Huxley didn't feel like keeping any of his cereal down. She thought she'd help him out with that.
They've been great puppies for Huxley so far. Soon, Huxley will be terrorizing them, pulling their ears and tails, and he'll probably make Spike pee on the floor. I don't think there's anyone who hasn't made Spike pee on the floor.
