To Interfere or Not to Interfere?

Okay, so Ben, Tyler and I are in the grocery store, and another family has a cute wobbly baby sitting in the cart seat, a tiny little thing. I was horrified to see a young woman (possibly mom) feeding the baby from a can of Mountain Dew, pouring most of it down her teenie chin. I saw this several times as we kept bumping into them as you are wont to do in grocery stores. I was horrified and didn't hide it except for whispering to Ben, trying to decide if I should say something. Generally, you don't interfere unless you see something illegal. I've often wanted to make comments when I see a parent yelling or spanking in public, and I once heard some head-spinning emotional manipulation from a mom to a teary-eyed preteen boy in an elevator once, but I've never interfered. But in this case, I was reeeeeeeally tempted.