Very young humans do not understand the concept of object permanence. They actually think you're gone, when they can't see you. That is also why you can sometimes see little children closing their eyes, when they are playing hide and seek.
Object permanence develops well before the ability to toddle, so Cillian here performs in the role of an infant.
Also, if you're just covering your face, they can see that you're still there, but not the part of you they actually connect with.