Hi Mary, Here is a link to one using command fridge clips.
Not sure that would be strong enough for little ones playing fort. Maybe tiring knots in sheets and using zip ties to wrap around the rails. How fun for them. I loved making tents and forts when I was little.
DIY Easy Bunk Bed Tent | Hometalk
You could use old bed sheets.
Paint on a design of a fort frontage, then use sticky velcro to fix to the top bed.