If you're staying at Caboolture, you're only really 25 minutes from Bribie Island. If you drive to Woorim, which is on the far side of the island, it's a surf beach where the kids could have a swim, or Bongaree/Bellara on the near side of the island are very calm and gentle (no surf).

There are also some nice national parks around the area, if the kids would be interesting in bushwalking/picnics - there are walks up near Montville/Maleny (about 3/4hr from Caboolture) and Kilcoy (also about 3/4 hr from Caboolture).