HI...
I just went through this with my 14 month old for his first.
I rented a fabulous area. IT is through palos park and it is their village green. It is on 123rd st a couple blocks east of lagrange. They have a pavillion outside that you can rent, and you hace access (for free) to a sand volleyball court, tennis court, baseball diamond, basketball court and a park for the little ones! It was great! We had over 80 people there and it didnt seem like it at all. All th ekids were entertained with something. They have picnic tables too....you just have to tell them how many you want. We just brought every different kind of ball, bags for the adults, etc and it was fun!
Check it out!
If you want more info...email me! Good luck and congrats!
B.