My ds2 went into the bottom bunk of his brother's (5) bed just before his second birthday. We put up rails and kept him in his grobag so he could wriggle around. He got out a couple of times at first but soon stopped.
We have done exactly that! And it has been a great success, no problems at all. DS1 aged 3.5 moved into the top bunk, Julyish. Gave him a few days to get used to it and he loved it, so then we moved DS2 then aged 20mths into bottom bunk straight from cot. We had a couple of nights where he kept getting out of bed, but we stayed outside the room and just kept putting him back in each time, no fuss, just popped him back in (ignoring the rage!) and said goodnight each time. Within a week they were both having a great time, they absolutely love the bunks, but not half as much as I do..!
Couple of points to consider - we did agonise for ages over this, and ended up buying a slightly smaller than normal bunk bed because they were quite young. They were also sharing a room already. Er... think that's it really. A few times DS2 has climbed up onto top bunk with DS1 - same thing, we just pop him back again. We have also instructed DS1 to tell us if this happens.
ooh, following with interest. strongblackcoffee, which brand of smaller bunk bed did you get? we have tiny room to be shared between DS nearly 3 and DD 6 months, once she reaches 1 and we want our own bedroom to ourselves. thinking of getting the ikea kura bed and having her on a kind of mattress on the floor, but seems wrong!
I trawled around for ages, finally got thuka shorty beds from here. But with the sloping ladder, and without the rope swing! I say we agonised for ages, because they were hideously expensive. However I am delighted with them, they're gorgeous and sturdy, the kids love them, and they're a lovely size. They also have the option to extend into full size bunks, singles, cabins, etc. (if we ever manage to save enough to buy the extra bits in the future)