Kalbarri has a number of parks that would suit the purpose and has river and surf swimming and fishing. Great attractions such as the Gorge and Rainbow Jungle Parrott Breeding Centre. Good shopping facilities but not a big town. Mrs Bravo and I spent some quality time there on our lap of Oz and it comes up pretty high on the I wouldn't mind going back there list.
You could also consider Cervantes, Jurien Bay etc.
Getting sites in school holidays could be a problem anywhere though, so make your mind up and get in early.