We spoke to several people who thought it might not be worth a day but for us, it was great. This may not be something the kids will love in a few years but they loved it at this time! The diamond area in the state park is a field in which diamond-containing rock runs seven miles deep. Every month or so they plow the field, exposing new soil and rocks. You pay an admission fee and dig as long as you want (all day! It's only 100 degrees!), keeping anything you find. There are diamonds found regularly - mostly yellow or brown but some white too. Half-carat diamonds were found right before our visit so they are definitely there!
You can walk along and look, dig in the furrows and do "dry sifting", or use the ramadas (shade!!) to do "wet sifting". Zack dug, Sammy roamed around, and I did wet sifting. It was more work than Zack was expecting and the weather was really hot but we all had fun.
By mid-day we'd had our fill of dirt and heat and not found any diamonds so we decided to call it quits and headed to the attached splash park and pool. Now, someone was really thinking with that pool attached to the hot, dry diamond field! That was at least as good as the diamond hunting. The pool was mostly two and two and a half feet deep - a great depth for our young kids. With the slides, fountains and play structures, it was a great place to play. Before this both Zack and Sammy had been really timid in the water, clinging to our necks or the side of the pool. But there the shallow water and beach entry gave them a chance to play without being nervous - I think they gained a lot of confidence that has been benefiting us going forward!