Hi S.!
Welcome to the area! There's a GREAT moms group on meetup.com called The Far Western Suburbs Moms Meetup Group. They have a couple of members in Yorkville, a bunch more in Oswego and then there's also some in Aurora, Naperville, Montgomery. I LOVE this group of moms, there's TONS of things to do. However, most of the moms have younger children...3 and under. But, for the summer, at least you would be able to get to know some of the moms yourself, and some of them do have older children like yours (not that they're SO old :) ) Also, the organizer for this group just moved here from Dallas recently, so you guys may have a lot to talk about!!!!
Here's the website for this moms group: http://moms.meetup.com/1000/about/
I live in Oswego and have found that this area (we're very close to Yorkville) has tons of moms and kids and lots of fun stuff to do. The Oswegoland Park District has lots of great activities for summer and they have a great pool too. I know Yorkville doesn't have a pool. But, the Oswego pool is the same price for residents or non-residents ($8.00, 3 and under are free). Here's their website: http://www.oswegolandparkdistrict.org/main_information.htm
I know you'll love it here once you get to know the area and some of the moms around here. I've found everyone to be very friendly!