Have just found your post. Don't know how old DS is, but if he's a little older, Beddington Park is great as there are sectiond of the River Wandle which runs through the park where you can easily get in. Parts of it are really shallow, other parts a little deeper - use your common sense! You need a change of clothes and a towel. My two always keep sandles on just in case of sharp things on the river bed. It's also good for fishing. There is a play area and a cafe' as well as a nice place for walking and feeding the ducks on the lake.