Hmmmm…what to do?

Went spent some time today at the new Splash Deck at our YMCA.  The boys had fun for the most part and Titus had a good time playing catch with another little boy he met there.  I, however, left with a little parenting puzzle I’m open to suggestions about.  Here’s the scenario:  Titus and his new friend were playing catch with our beach ball and when it went into the grass, they quickly ducked under the “caution” tape that is still up because they haven’t even gotten it taken down with it being so new.  I still figured it would be good for Titus to respect the tape even if it wasn’t an actual danger to him so I asked him to stay on the other side of it and he complied pretty quickly.  The other little boy stayed in the grass and began pulling on the caution tape until he ripped it.  Titus sat there rather patiently and slightly irritated because the boy was still holding his ball and he knew I wasn’t going to let him go and retrieve it.  The boy’s Mom (who I have never met) was on her lawn chair reading and not aware of what was happening.  I never know how to address things with Titus when I ask him not to do something that another child is being allowed to do.  It certainly wasn’t a big deal today but it did get me thinking that I wish I had a good way to handle similar issues for the future.  Any wisdom fellow parents??