I read this tip a long time ago and have used it in many situations since then. Memorize this magic line, "Sorry, I know I'm a totally overprotective mom, but...."
Am I an overprotective mom? Nope. Not that I know of. In fact, I would say I err on the side of underprotective. :) But it works in so many situations where you feel the problem is the other mom's kid, but you don't want (or need) to say so. It puts the blame onto yourself, (hopefully) sparing your friend's feelings
You can use this in situations where you don't agree with the other mom's carseat use (or lack thereof), when someone else's kid is a bully and the mom doesn't seem to know or care, a party where you don't feel there's going to be enough supervision, and so on.
Try it out next time you're stuck in an awkward situation with a friend!