My first response was panic.
My second response was to pass the buck, "Go talk to your mom about that," I said.
She ignored me and said again," I just wish these were bigger."
"Man up," I told myself, and knelt down. I said, "When you're ten or eleven, your body will start to change. And one of the things that will happen is that those will get bigger. Lots of things will happen - you'll get taller, for example. But right now, you're still working on being a kid, and you're going to keep learning and growing for years yet before that happens. Now, do you want to wear you're supergirl shirt today?"
Ellie raised her hands into the air and shouted, "yeah!"
That's the best I could do on the spur of the moment. What do you think?
As a related aside, I know that "man up" is a contested phrase, evoking machismo culture and other unfortunate aspects of masculinity. That's why I like to repurpose expressions of masculinity like "man up" to wiping bottoms, getting the kids ready for school, or in this case talking to my daughter about her breasts. It's about expanding the range of normative masculine/paternal behavior.