As a general rule, most children need a break when they first get home from school. Fifteen to 20 minutes for a snack and a talk about their day can help them to unwind from the hectic pace of school, and then they'll feel rested and ready for homework. The break can be extended if they need a little more down time, but if you wait too long to get started, homework can move into dinner and bath time. For most children, the ideal is to have a break when they get home, do homework, then have some time to relax before and after dinner.
As the new school year starts, you can try different schedules for a week at a time until you find one that meets the needs of your children.