Let me first say that Poltergeist is absolutely in no way a film for a seven-year old, and certainly shouldn't have been rated PG. I say that as someone who loves horror films, and also found scary movies/horror in general an incredibly cathartic tool to overcome some pretty severe traumatic childhood stuff. But that kind of thing is based on personality and temperament and definitely isn't going to be for everyone. Regardless of all that, when you're seven, you should be watching E.T. The Extra Terrestrial, not Poltergeist.

Here's an interesting fact about the rating of Poltergeist: In the US, it was released before the advent of the PG-13 rating, and as you point out, got rated PG instead of R. That's the same rating E.T. The Extra Terrestrial received.

Here in the UK, things were very different. E.T. The Extra Terrestrial was correctly released with a "U" certificate - the lowest rating, meaning it is suitable for all audiences. Poltergeist, by contrast, was released with an "X" certificate, which was the highest rating a film could receive. So in the US, the films had the same certificate, and in the UK, they existed at the opposite end of the certification spectrum.

Since then, the UK rating system changed. Poltergeist now has a "15" rating ("18" is the top rating in the UK these days), and E.T. The Extra Terrestrial remains in the "U" category. But there are still several ratings between them. U, PG, 12A, 15, and 18 are all the UK categories, and in 1982, just before UK film certificates were changed to the present system, there was U, A (like a PG), AA (under fourteen not admitted), and X (under 18 not admitted).