Close Encounters of the Third Kind is rated PG, but it's practically a G in our books. We're guessing the rating bump was likely due to situations that can be over little ones' heads and some potentially frightening scenes resulting from miscommunication—why the aliens thought they needed to destroy Jillian's kitchen to say "Hello" is beyond us.
There are also no sexual situations to speak of in Close Encounters; it makes the innuendoes of Disney animated movies look like Porky's by comparison. The closest we get is Ronnie reminiscing of when she and Roy would drive to hilltops to "snuggle" and a scene where Roy's boys play a prank on him while he brushes his teeth. So unless you are particularly sensitive to talk of spooning or knowing that Richard Dreyfuss is a boxer man, you should be fine.