In NSW at least a child who is born in december has to start school the year after they turn 5 as they have to attend school in the year they turn 6. So for you baby, she would have to go to school just after she turns 5, but considering her birth etc, if there are still big enough delays by the time she is that age she may be able to stay back another year. However she would be too young to send her just after she turns 4 regardless of any developmental stuff.