I covered this in a Shakespeare unit at university (BA English Lang & Lit), but have never actually seen it on stage. I don't think it has been staged that often, compared to some others. My tutor at the time, though, said that when it was done at the National, the St John's Ambulance had to take people out who had fainted - so I don't think it is just to do with standing in the heat. I don't think anyone has mentioned the heads in the pie - which is served to the mother of the people to whom the heads were previously attached. Is someone also toasted on a grill? Or I may be confusing that with another revenge play...
I think it's probably a bad idea, primarily because they haven't studied it and can't therefore contextualise what they're seeing and discuss how that might affect the audience. And debrief afterwards. And it's not a particularly good play: it's important, I think, primarily as a signpost in the development of his style.
The only reason I can think of for going to see it at this age is because, as an early play, the characterisation has more in common with Jacobean tragedy than his later plays - and is therefore more two-dimensional (in my opinion). It's less emotionally involving. One might develop a more intense relationship with the Macbeths, for instance, or the characters in Lear ("Out, vile jelly!" as the eye pops...) than with anyone in this play.
I don't believe in age restrictions on the Arts because (from my own experience) I think people take from things what they are emotionally capable of drawing. That said, I would not take my children at that age to see this as a first choice - and if, by some bizarre twist, it was compulsory, I would sit down with them first, with the text and talk it through. And then see it with them.
Funny thing. Twenty years ago I would have rushed to see this. These days I've learned to love the comedies...