Timothy Johnson Park is in Watertown. It is a fairly generic park, but we had fun there.
It was well-visited while we were there. It definitely wasn't overrun with children, but I get the idea that when you are there playing, chances are you will get to interact with other kids too.
Timothy Johnson has two climbers, one designed for young children, one for older. There is also a nice covered picnic shelter with bathrooms and a bubbler!
Timothy Johnson Park
1115 Oak Street ~ Watertown, WI