Do I need blocking between 2x8 joists with an 8-foot span in a kid's fort?
Not really. 8 feet is a fairly short span, and you're unlikely to see substantial movement even with some warpage. It certainly won't be enough to reduce the capacity of the floor to a dangerous degree. (No deck I ever recall seeing has had joist blocking.)

Minor point of clarification: Blocking doesn't reduce cupping (curvature of lumber across the grain). It prevents displacement (curving) of the joist along its length. That's not typically referred to as "cupping". 

One further suggestion... Typically flush beams (rim joists acting as beams) are sized larger than the common joists because their load is much larger. I'd either double yours, increase the rim size to 2x10, or reduce the common joists to 2x6, depending on your stiffness standards. 2x6 joists on 16" centers are adequate for a playhouse of that size, IMO.