We wanted a short bunk bed for our sons, age 4 and 1. Our 4-year-old insisted on a Thomas Train bed. We modified the fort bed plans slightly to add stronger barriers on the top bunk and add a barrier on the bottom for the little guy. We also used solid plywood rather than MDF. We used 2x4's and steel angle to hold up the top bunk. It should be sturdy enough for them to use into teenage years. Because we beefed up the strength, the cost increased as well.
We tried staining it and it didn't come out as nicely as we had hoped, but the boys added their own stickers to it and now they have a Thomas bed that they love. When the 1-year-old gets older, we will probably remove the barrier on the bottom bunk to make getting in and out easier.