I used to be sign language interpreter at a community college, and I interpreted in health classes occasionally. The sign used for breastfeeding was (hard to describe in words): hold your dominant hand palm up over your breast, touch thumb to pinky, then quickly slide thumb forward across fingers while closing your fist. Thumb will end up pointing foward, while fingers end up folded onto your palm. It's a quick movement, kind of like snapping your fingers - but it's a really a slide, with no sound. You can do it over just one breast, or quickly do the sign over one breast and then the other. I did it the second way in the more formal college classroom setting.
There may be other signs for it, too. You could also make up a family sign for it. Just like hearing children make up their own words for things that become special words in a family, deaf children do too!
We were posting at the same time!
Amanda gave a great example of a family sign.
Laura