I am bringing my DD nearly 5 and DS nearly 3 year old in August. My DD is mad into WofO and the 3 year by association. Have your boys seen the film. I think they'd need to be familiar with the story and not too scared of the flying monkies. But its a great stories so bet they will enjoy. I am really looking forward to it!
I took mine (8 and 3.5) last week. TBH it was a bit of a disappointment. Everything seemed in place: costumes, music, scenerey etc. But I just didn't like it. The DCs found it a bit tiring, but we'd been to the Motor Show earlier, so it was a long day for them. They liked the music, though, and DD has named a dolly Dorothy since then and a stuffed dog Toto, so she must have got something out of it too.
I got the tickets through Lastminute and paid £20 each, plus booking fee. Should have tried directly from the venue as only £12 for children. Silly me.