Should little kids be banned from R rated movies even with parents?

I swear every time I go to an R rated movie there is some 4 or 5 year old screaming and terrified, or sitting there with his little eyes bugging out watching a graphic sex scene. There are usually more kids in the violent and scary movies though ... I guess that's more appropriate than sex for whatever reason?
Update: Here it means anyone under 17 requires a parent or legal guardian. Seems like the parents aren't really looking out for the kids though. I don't know what moron decided that - does it suddenly become ok for a five year old to see someone get their head blown off just because mommy's there?
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