Sometimes a natural bond develops between older and younger siblings. If not, you might want to help things along by pairing them up yourself.
Having an older child take charge of a younger one, teaches them responsibility and nurtures sibling relationships. It also saves you a lot of time! The older child can help their "little buddy" with her homework, set up her meals and get her washed and dressed in the morning.
You also might want to let your older children earn some money by watching the younger kids now and then, so you and your spouse can have an evening out. However, the time older children spend babysitting their little brothers and sisters should be limited. You don't want the older child to resent the younger ones or feel tied down by them. Developing a natural feeling of responsibility for others is your goal.