Simple little things like explaining the basics of the EKG monitor helps quite a bit. We all know that a change in psoition or movement can change the monitor, but many parents do not. Even the basics about the vent if the child is on one. Besides helping alleviate their fears, though not all, it also shows them that they can trust you for caring for their loved one as you know your "stuff." This works well in PICU, NICU, as well as all adult ICUs.
Good luck to you.