HI L B
We enjoy our passes to the Gurnee Aquatic Center. There were some days that there was a lot of people there but I have never felt like it was overcrowded. We spend most of our time in the zero depth and shallow area so this is the area I am commenting on.
There is a sand pit right in the pool area. My kids enjoy playing with the buckets, faucets and sand and I enjoy that there is a shower right at the exit.
The pool opens at 11 for season pass holders.
Is it worth it? Because we had the pass, we went more often. If you want to save money, consider getting the pass just for yourself and your son(he's free if he is under 3 at the start of the season). Also, be sure and take advantage of the early bird rate. We had considered this but decided to get the family pass for all of us and it worked out well. My husband did not go near as often as us but he joined us a lot more times after work (and occasionally weekends) since he did not think of having to pay the day rate.
I have not used the Grayslake pool but I have heard nice things about it and that it is not crowded. There is a splash park close to the Grayslake pool and it is free.