Tried to send you a private message, but for some reason can't? Can you send me one with your email address and will answer you in more detail.... To briefly answer your questions, we have kids, but much bigger than yours, schools have excellent reputation, most kids ride to school including the 'little ones'. Beaches about a 10 min drive away.
My sister lives next to Mudjimba, lovely area, one drawback is you have to cross the Maroochy river to go to Maroochydore, and the traffic queues to go across in rush hour and other times during the day Having said that they are widening the bridge as I type!
We have lived in Sippy Downs for nearly 3 years.
PM me and will fill you in with more details.......