I have done a couple of different ones, sorry no pics.
First time: went to part surplus section at local RV dealership and bought a section of the aluminum ladder for the back of rv's. I used a hack saw and cut the exact height (really tight fit) between the two bunks. I believe there were three rungs on the ladder when done. I then had a enough overhang on the top bunk I used emt straps (metal straps shaped like a "C" and secured the top and bottom. The ladder itself rested on bottom bunk and the straps just kept it from moving.
The second time I had a twin over full. On the foot of the bed I just bought a small sturdy shelf from a home improvement store and put it about 1/2 up the wall. They used it as a step to help them up top. Depending on age of kids this might work.
2012 Freedom Express Liberty Edition 292BHDS
2011 GMC Yukon XL 2500 4X4 (what a difference)
2007 Ford Expedition EL Eddie Bauer 2WD (sold)