First off, I agree with everyone else, thank you for caring!
Like Freckles suggested, your best option might be to approach the parents and ask them if there is anything you can do.
I had a slightly similar situation- I was with DS at the children's hospital, cafeteria, buying lunch. It was pretty chaotic, I picked the wrong time to go in there. We were in line to pay, and I had a backpack, a tray of food, trying to dig out my debit card, and keep one hand on DS, who was having a meltdown. I honestly wanted to have a meltdown right along with him.
This kind woman behind me in line asked me if there was anything she could do to help. While I knew she couldn't handle DS's meltdown, she could hold my tray for a sec while I dealt with him. And that was a significant thing at the time.
I went to a Parent Training at Judevine, where they teach a version of ABA. What they had us do, as parents, if the kid starts to tantrum, is to 1) make sure they can't hurt themselves, but 2) ignore the behavior while at the same time doing something really cool that might distract them. That wasn't in crowds, though. But it might be worth a shot. Don't try to directly interact, do "parallel play" sort of, close enough for the child to notice but not so close that they'll feel like you're in their face.