This came up on Duolingo a while back and I'd like to get the community's consensus.
The phrase they give for We are playing cards is Ni ludas je kartoj. So would je be appropriate when you are playing with a particular item? Example: My kitten plays with a string => Mia katido ludas je ŝnureto
If not, what would be more appropriate? I would think kun implies participation of the item being played with (Mia filino ludas kun sia frato) for instance, although in the case of my kids this could either have been they were playing on the swing in the back yard, or she was dressing him up like a Barbie doll.