Megan Fox is a terrible actress! No! Say it ain't so!
Also, Dusty, you and Shia... who would've thought. Perhaps it's the monkeys.
As for the gruesome death -- I actually think I'd be amazed more than horrified to see it as a 5-year-old, especially if my dad insisted he bring me to all those big movies you mention. It's at the tail end of the ants sequence, which is actually amazing. So put two and two together and figure out what happens -- this thing is a bit more prolonged than the old mauled-by-propellers or crushed-by-rock-crusher bits. It might be as cool as the melting heads, though.
The thing is, the final half hour of this movie is such eye candy that the kids will just be staring at the screen in amazement, regardless of their ability to comprehend the purpose of it all. (It also flies over the head of half my colleagues. Their fault for not reading up on their occult lore.) Also, compared to the spectacle this film ends with, all other Indy films seem small.
It's still not the best film, as Raiders is shot for shot and line for line perfect, but other than that, it's the best time you'll have at the movies this year.