We bought bunk beds when my ds was 4 and dd was 1.5. We put ds on top and moved dd out of a crib to the bottom. They stayed that way for about 6 months and ds started not wanting to be on top. We unbunked the beds which took up a large majority of their bedroom. Last weekend (about 6 months later) , they started saying they wanted them bunked again. But ds (now 5) did not want on top (he's afraid of heights so that could be the problem), so we put dd3 (turned 3 in December) on top. I was a little worried about her getting down to go to the bathroom in the night so I put a nightlight in the outlet right next to the steps. She's doing great and loves her bed up there. I can't say I wasn't a little nervous and I still am when I see her walking around up there. Hopefully, we never have a fall from there. By the way, the label imprinted on the bed says not to put a child under 6 on the top bunk.
T.