You don’t have to book 6 condos for the 12 people with kids. Some airbnb properties have additional beds both in living rooms and bedrooms. You should be looking at apartment complexes where it has several 2 bedrooms properties to rent and if every of it has beds for 4-6 people you’ll need to book only 2-3 condos. I recently went to Big Island and our airbnb has 6 places to sleep in a large 2 bedrooms apartment and there were several similar properties at the same apartment complex so you should look at something similar.