For your suggested budget and lifestyle requirements have a look at the Sandgate/Deagon/Brighton area. The Moreton Bay foreshore there is ideal for kids to cycle, swim & play. Great children's playground park at Shorncliffe. Right on the beach. A bit further out there is the Redcliffe peninsula. Has great foreshore amenity, sandy beaches, schools and shopping. Your rent spend is a bit tight but I am sure you will pick up something reasonable for that.