LIzzer we have one for our 7 year old son, but it has a metal frame. The top bunk is a single bed and the bottom a double, that can be converted into a sofa. It is really useful, son loves it, and it was, at £370, definitely a good buy.
My son soon got over the novelty of sleeping on the top bunk and adopted the double bed. He liked the space - and most importantly, he wanted to be close to his bedside table and nightlight.
This is one problem with high bunks especially metal ones. How do you attach a safe, child-proof light to the upper bunk, apart from fitting a wall light? Clip-on lights are rather dangerous, I think. A young child could easilty pull one off and then the hot light bulb accidently lying against bedding could become a fire hazard. Other mumsnetters may have a good answer to that.
That problem apart, a great advantage of having a convertable double bunk below, is that it becomes a spare bed for my husband and I when we have our in-laws to visit. MIL and FIL can have our 'posher' bedroom, while we can easily fit into our son's room. We have also used the sofa bed for putting up guests. It all depends whether our son and the guest make happy room mates.
I can't recommend a good source of pine beds, but there are lots out there, so I'm sure you'll get lots of responses.