At O'Hare Airport, there is a really cool play area just past security in Terminal 2. It's big enough that kids can run around, climb model pastic airplane, use slides, and just be kids.

If you are flying from Terminals 1 or 3, then after you pass security for your own terminal, simply follow signs to Terminal 2 and you'll see the child play area. From terminal 2, there is also an airside shuttle to Terminal 1 Concourse C.