We attended a party this summer at Traxler Park in Janesville. It was a great venue, and we had a lot of fun!
The bathrooms were pretty nice. I assume they are usually open, but it may just have been for the party.
One note of caution: this park is on the water, so it's something to keep an eye on, but the playground was not close enough to cause us much concern. There is a water-skiing team that performs at the park that would be pretty fun to check out too.
600 N Main St ~ Janesville, WI ~ (608)755-3025