Hi eeww,
You need a permit to fish in Wivenhoe. and they will fine you if you don't have one.
But you can fish below the dam wall without one. If it is close it is not a bad place to catch a few tiddlers with your son. There is a road that goes to a lookout below the dam wall. From the look out you can see an almost round pool downstream. You can park down near there and walk down the hill to there. Worms will work best , fairly small hook. There are usually heaps of small spangled perch in there, can catch them and let them go for fun.