I have been here several times with my 3 year old. They have a children's garden that he enjoys playing in. However, he doesn't enjoy it anymore than a free playground. They have several walking trails and a nice drive around the park.
The cafe is actually very good. They have healthy options that actually taste great. I would go just to eat.
If you are a couple I could see this place being a nice walk or date. However, if I were short on money or just looking for a place for the kids to roam...I would go down the road to Cantigny.