Damn straight! Plus, kids in the audience of a horror movie are distracting. It’s poor parenting and just fucking rude. R-rated horror is for Grown Ups, silly rabbit!
Different kids have different responses. Some little children will be absolutely terrified of something as seemingly innocuous as the golden- and silver-age Universal horror movies, others will have no problem dealing with something like Romero zombie movies (I know I didn’t by the time I was six years old). The only hard and fast rule parents should follow on an issue like this is: know your kids, communicate with them, and understand what they can and cannot handle before making decisions for them.