nah- i wouldnt say anything- their parenting choices, and although you know that its wrong, it is up to the parent to make sure their child is restrained properly.

Look at the other point of view- how would you feel if someone said something to you about how you had your child restrained in their car seat??
If it is really getting to you, why not post something to the general audience on your wall so that the person has a chance to see it. even tag a few of your friends with kids in the same age bracket so that they now its been posted