Prepare for the worst: they are not truly in control of themselves yet and relying on them to never mess up is not good practice.
Structure a shooting schedule around their abbreviated availability. Kids do not want to be on set for 15 hours.
Arrange all shots featuring the children to take place first, with different crew and cast calls for others.
Have a backup schedule in place in the event that a child actor needs an unexpected break. This way you don't lose any time, because time equals money in the world of film.
Be extremely patient with line flubs. However, also keep in mind that children who constantly mess up or lack focus do not belong on set.
Step 5 ties into the auditioning process. Do not settle, because the wrong actor can hold up a film.
Have fun. Kids usually respond best to fun, jokes and pranks, and they need to develop trust that comes with friendship in order to succeed.