What to say about baptizing?

My husband and I will not be babtizing our future son. Both of my grandmothers are extremely Catholic (my grandfathers are dead). My aunts and uncles are also super religious, but I'm not concerned about them. My parents are super supportive of my atheism.
 My grandmothers are old, sweet, and will be heartbroken when we don't babtize. I'm at a loss for what to tell them when they ask when the Babtism will be. Any advice?