I'm really enjoying this thread, though I'm surprised that no one has yet mentioned Dimetapp.
How about bringing along a bag of props: toys, blocks, books, things with removable parts, anything that if thrown at you wouldn't hurt much? I have used those plastic easter eggs with little toys inside (not jelly beans) - kids like to open them up. Or dress-up clothes? (This last one will work if your client can think outside the box.) I personally like small child portraits where the child's attention is focused on something other than the camera.