FirstBornTriplet
How does a second grader even know such site's exist, let alone hot to access them? When I was that age the only thing I was interested in was games.
Most 7 year olds know what boobies are.
Keep in mind that with current search engines (Google etc) they do not have to know sophisticated search terms to stumble upon porn sites.
They could simply type in naked or breast or booby or really virtually anything that is a word for a body part or nudity and some of the hits will be porn sites.
Most 7 year olds do not have sophisticated knowledge about sexual intercourse or weird fetishes but most of them are old enough to have at least mild curiosity about the human body and the opposite sex. There is a decent chance they didn't start out specifically looking for a "porn site" but if Johnny stumbled upon one there is a decent chance he would tell Bobby, "Hey, look what I found!"
Also keep in mind that some people call just about anything porn these days.
Like that recent Sports Illustrated swimsuit cover people were calling porn.
There is a decent chance they did find "real" porn but just about anything would have some parents up in arms.