I've just ordered a replacement metal bunk bed from IKEA for £95. I went to see it first and it's quite solid. Re GC staying over, I only change the sheets once every 8 weeks or so as, if they stay one night a week, that's only 8 sleeps.
'Officially' it's not recommended that children under 6 use a top bunk, but it very much depends on the child. Of course you need to make sure the mattresses are not too deep or they might nullify the function of the guard rails. And I had a rule that no-one was ever allowed to play or climb onto the top bunk, except for sleeping.
Most accidents I think are due to kamikaze antics as mentioned in some posts already which is why I always have a monitor turned on when they are playing in the bedroom, so I can hear what they're up to.