Welcome to McHenry County.
Here are some ideas:
Woodstock has a great pool "The Water Works" that is just like a water park. Corner of Rt. 14 and Kiswaukee Valley Road. Adults ###-###-#### (Rec. Dept. phone number)
There is a "Randall Oaks" a petting zoo / park on Randall Road outside of Elgin. ###-###-####
Crystal Lake has that great park/playground on Crystal Lake Ave.
There is the bike trail that starts way up by Richmond, runs through Crystal Lake and goes down past Algonquin. But you can bike with your 5 year old and the little one in a seat behind you.
You could have fun picking strawberries at the strawberry patch on Kishwaukee Valley Road, between Woodstock and Marengo.
There is Magic Waters in Cherry Valley outside of Rockford www.magicwaterswaterpark.com ###-###-####
There is the McHenry County Conservation District. They have events all over Mchenry County through out the year for all ages. http://www.mccdistrict.org/ They have classes kids can sign up for or family events like "Trail of History" and much, much more CHECK IT OUT !!
Check out his website http://www.visitmchenrycounty.com/navigation/outdoor.html#
The YMCA has day camps for your 5 year old.
Hope some of this helps you. Have a great summer
S.