I have been to all of those islands except Brac. I am assuming the 5 and 12 yr olds are the nondivers. Of all the choices, Provo has the best beach by far of all your options so if you are beach people that would be my recommendation for an island. One of the dive knocks on Provo is long boat rides. They can be long but I find it worth it and they don't have to be. Dive Provo's 1 tank PM off Grace Bay is a couple hours and the diving is fine. A two tank off Grace Bay will likely take all morning to 1 PM ish. Check Dive Provo's website. I don't know anything about the resort based dive op.
Curacao has decent beaches as well and very good diving and if you are looking to stay at a resort I would recommend taking a look at Lion's Dive Resort.
Little Cayman may be too quiet for the family and limited dining and shore activity options. I would save LC for a dive dedicated dive trip.
Bonaire is also a good choice but I would wait until the kids are ready to get certified. There is no place better for kids to start diving IMHO.
Whatever you decide, enjoy your trip.