You can get cheap flights to Hammo at times and sometimes with all the transfers from Proserpine it can be cheaper and also a lot easier. Hammo is great for families. You can go through Hamilton Island Enterprises and stay in either in one of their hotels or a Whitsunday apartment (gorgeous views and self contained) plus a few other self contained options, or you can stay in private rentals like townhouses/villas which usually come with a golf buggy included. The real estate or booking agent on the island can help you there. If you stay in the HIE accommodation then as a PP said kids stay and eat free. There is good occasional child care available and they do great activities with the kids.
Depending on the time of year the price will probably vary alot.

Daydream is the other kid friendly island but I haven't stayed there.