Do NOT give my 3 month old coffee!

I see posts on here all the time about people's families giving their babies food/drinks they aren't supposed to have yet. I was so glad my family wasn't that dumb. I was wrong. DD, DF & I were at the mall and ran into MIL & SIL at the Starbucks. We were talking and MIL was saying to DD"do you want some coffee? You do don't you?" I didn't think anything of it because I say stuff like that too, only I'm just kidding. She apparently was NOT. She stuck her finger in her drink and shoved it in DDs mouth so quick. DF yelled" don't give her coffee" her response was "it was only a little bit" so at this point I'm very pissed off so I pushed DDs stroller and we walk away. Then after I'm walking away DF said his sis and mom were confused why we were upset because "it's not like they gave her a lot" I'm so mad!