How young is "too young"?

Okay, so I saw Watchmen last night. fantastic movie! Very well done. However, that's not what this entry is about. It's about something a little different. It was me, my boyfriend Dennis, and my friend Krystal were sitting in the movie theater, waiting for the movie to start. A couple sits next to my friend Krystal, normal right? That's until their two children came running up behind them. They couldn't be older than seven or eight. All I could say was "Do they know this is an R rated movie?" So, as the movie was going on, all the murders, the blood and gore, and the sex, all I could think about was those two kids. This is not a movie they should be seeing at this age. After the movie was done, my friends told me they felt the same way I did, and joked about how awkward of a car ride home it had to be for those kids. Which brings up my question, was it bad for the parents to bring their kids to see Watchmen, knowing that it would be an R rated movie? Would you do the same thing?