I doubt he saw it. Reason 1} He would not have waited for her to walk to a park and sit down before trying to talk to her (stalker Ren??) Reason 2} Ren knows Kyoko would go to a place similar to the forest when she cried over her mother, so it's not unreasonable to think he could find her in the nearest park to her home.
quite the contrary for me he saw it and he waited to see kyoko reaction to the kiss (which she handled brillantly) and he always leave Kyoko and Shou to their world, he even said that himself before they have a world of their own (also this world tend to disappear lately) anyway you can also add the fact the panels with the car passing by when the kiss occured, Im sure Ren was in it!!
As for the park I think he followed her after the kiss..
Again its just my supposition lol
Ok im gonna ask your opinion since the chapter is out... Did Ren see the effing kiss or not? Cause I mean the kiss took a lot of panels to show, and there is that car/taxi passing by... what do you guys think?