Playhouse or shed?

[Maybee]

Hi, I'd love a den for my 3 boys aged 8, 3 and 1. So it needs to be a good enough size and not too babyish. My budget is up to £250. Would a garden shed be the safest bet?
Ideas anyone?

Possibly for longevity yes - DS refused to set foot in the playshed after about 10 as it was "too babyish"

thanks i think i'll look for a shed that is easy to put up if possible