How do you keep a straight face?

Cailin has been in the habit of kicking lately. Not when she is standing, but more when she is laying on the floor in front of me while I change her diaper. It hurts. For those of you who don't know, one of the first signs that you are pregnant is that your boobs hurt. They are just overly sensitive. When a size 6.5 little foot kicks you in the boob, or in the gut, it hurts. I have tried grabbing her leg and holding it sternly and telling her NO. I have tried telling her that it hurts, I have tried everything I can think of. Today, I told her there was a baby in my tummy and we needed to be easy. She still kicked. I asked her if I needed to spank her bottom for kicking me. She rolls over, points to her bottom and says "Ere'tis". Thanks. At least now I know what I am going to swat.
It was all I could do to not laugh. I could not however hide a smile. Anyone have a good idea how to put an end to this kind of thing?