there used to be a circus school in temple bar on either a sat or sunday morning- meeting house square I think.
Check out the ark also in temple bar- loads of great stuff for kids
http://www.ark.ie/index1.html
The zoo is brilliant and something for kids of all ages- also in phoenix park you can used wander play ball games or sit around by Farmleigh house where there's a pond and a cafe
http://en.wikipedia.org/wiki/Phoenix_Par…
The national history museum on merrion sqaure is also fantastic for kids
http://www.museum.ie/naturalhistory/
and loads of free events go on during the summer just ahve to go looking for them
http://www.dublinks.com/index.cfm/loc/9/…
also good is simply feeding the ducks in stephens green- there's a pretty good playground there, if its playgrounds you're after go to Marley Park in Ballinteer as well and Tymon park in south dublin
have fun