Blue tits nesting

Hi I have a pair of BT nesting in a bird box on the side of my shed. They are feeding there chicks well and I have a feeder station with seeds coconut mealworm and the usual feed plus fresh water, I keep it replenished everyday and the BT like to feed the chicks the occasional treat from the food supply. We keep a close eye on the box to make sure nothing attacks the nest and at the moment all seems fine. I have a few questions about the next box. When the birds leave do I clean out the box ? If the chicks don't survive is it possible that the adults might breed again straight away? Do the adults sleep overnight with the chicks? Any other advice would be a great help in taking care of the nest box,it's the first time they have nested in the box which has been up for three years . Thanks Paul