What is with the utter fear of confrontation? Is it just something in the feminine make-up that abhors it? Sorry, just a guy thing here. I'll just chalk it up to something else we just don't understand about the opposite sex, albeit our "list" is beginning to get as long as Santa's...
The trick is constructive criticism. Tell them about the things they are doing right, or at the very least how cute their kid is if they aren't doing anything right. Then tell them what they are doing wrong and why, and then offer a way, or ways, to correct the deficiency. Teach them how to fix the problem on their own if it happens again. Then praise them again for learning how to correct the problem.
Ie... I see that your child isn't buckled into the seat, but you are certainly using the retaining clip correctly! Cute kid btw. Aww... Gitchy gootchy, etc. Did you just unbuckle the straps for the walk in/out of daycare? Yes? Yeah, dealing with those bulky snow suits can be a real PITA. That's why I don't use them. (Insert discussion of thin fleece and blankets, car seat poncho's, and shower cap style covers here. Give her a cheap shower cap style cover if you want to buy one in advance to make both of you feel better and to engender the warm fuzzy's. This is also yet another good time to talk about fabric compression.)
No? Then you really need to have them buckled while in the car. The retaining clip is meant to break in a crash, and is just there to position the straps correctly in case of an accident. That means that if you are in a wreck, it is very possible that your child could be ejected from the seat, and we wouldn't want that would we? So, how can we fix it?
Let's see if we can loosen the straps some. Yes? Good, that works. (Insert discussion about compression here if you want, as you've already broached the subject of her child being thrown from the seat in the event of an accident.) No? Then we need to get her out of that bulky snow suit. Yes, there are ways to keep her plenty warm in the car without it. (Insert discussion of thin fleece and blankets, car seat poncho's, and shower cap style covers here. Give her a cheap shower cap style cover if you want to buy one in advance to make both of you feel better and to engender the warm fuzzy's. This is also yet another good time to talk about fabric compression.)
Get child buckled in correctly. Show Mom the correct placement of the retaining clip (tickle zone), how tight the straps should be, and how to adjust them. Make Mom show you she can accomplish these tasks. Praise her on how well she did them, how safe her child now is because it's so obvious that her Mommy loves her so much, and isn't he/she the cutest thing!
There, you've praised, you've constructively criticized, you've offered solutions, you've taught her how to apply those solutions, and you've praised once again. But... All of this hinges upon your ability to confront the issue in the first place. KWIM?